try out putting block states on slopes instead of blocks

src/main/java/io/github/cottonmc/slopetest/block/SlopeTestBlock.java

@@ -10,6 +10,7 @@ import net.minecraft.entity.player.PlayerEntity;
 import net.minecraft.item.BlockItem;
 import net.minecraft.item.ItemPlacementContext;
 import net.minecraft.item.ItemStack;
+import net.minecraft.item.ItemUsageContext;
 import net.minecraft.state.StateFactory;
 import net.minecraft.state.property.DirectionProperty;
 import net.minecraft.state.property.Properties;
@@ -58,13 +59,15 @@ public class SlopeTestBlock extends Block implements BlockEntityProvider {
 	@Override
 	public boolean activate(BlockState state, World world, BlockPos pos, PlayerEntity player, Hand hand, BlockHitResult hit) {
 		if (world.isClient || !(world.getBlockEntity(pos) instanceof SlopeTestEntity)) return true;
-		if (player.getStackInHand(hand).getItem() instanceof BlockItem) {
+		ItemStack stack = player.getStackInHand(hand);
-			Block block = ((BlockItem)player.getStackInHand(hand).getItem()).getBlock();
+		if (stack.getItem() instanceof BlockItem) {
-			if (block.getDefaultState().isSimpleFullBlock(world, pos)) {
+			Block block = ((BlockItem)stack.getItem()).getBlock();
+			if (block.getDefaultState().getOutlineShape(world, pos) == VoxelShapes.fullCube() && !(block instanceof BlockEntityProvider)) {
 				SlopeTestEntity be = (SlopeTestEntity) world.getBlockEntity(pos);
-				if (be.getRenderedBlock() == Blocks.AIR) {
+				if (be.getRenderedState().getBlock() == Blocks.AIR) {
-					be.setRenderedBlock(block);
+					ItemPlacementContext ctx = new ItemPlacementContext(new ItemUsageContext(player, hand, hit));
-					if (!player.abilities.creativeMode) player.getStackInHand(hand).decrement(1);
+					be.setRenderedState(block.getPlacementState(ctx));
+					if (!player.abilities.creativeMode) stack.decrement(1);
 				}
 			}
 		}
@@ -91,8 +94,8 @@ public class SlopeTestBlock extends Block implements BlockEntityProvider {
 		BlockEntity be = world.getBlockEntity(pos);
 		if (be instanceof SlopeTestEntity) {
 			SlopeTestEntity slope = (SlopeTestEntity)be;
-			if (slope.getRenderedBlock() != Blocks.AIR) {
+			if (slope.getRenderedState().getBlock() != Blocks.AIR) {
-				ItemStack stack = new ItemStack(slope.getRenderedBlock());
+				ItemStack stack = new ItemStack(slope.getRenderedState().getBlock());
 				ItemEntity entity = new ItemEntity(world, pos.getX(), pos.getY(), pos.getZ(), stack);
 				world.spawnEntity(entity);
 			}

src/main/java/io/github/cottonmc/slopetest/block/entity/SlopeTestEntity.java

@@ -1,9 +1,10 @@
 package io.github.cottonmc.slopetest.block.entity;
 
 import io.github.cottonmc.slopetest.SlopeTest;
+import io.github.cottonmc.slopetest.util.BlockStateUtil;
 import net.fabricmc.fabric.api.block.entity.BlockEntityClientSerializable;
 import net.fabricmc.fabric.api.server.PlayerStream;
-import net.minecraft.block.Block;
+import net.minecraft.block.BlockState;
 import net.minecraft.block.Blocks;
 import net.minecraft.block.entity.BlockEntity;
 import net.minecraft.nbt.CompoundTag;
@@ -12,31 +13,31 @@ import net.minecraft.util.Identifier;
 import net.minecraft.util.registry.Registry;
 
 public class SlopeTestEntity extends BlockEntity implements BlockEntityClientSerializable {
-	private Block renderedBlock = Blocks.AIR;
+	private BlockState renderedState = Blocks.AIR.getDefaultState();
 
 	public SlopeTestEntity() {
 		super(SlopeTest.SLOPE_ENTITY);
 	}
 
-	public Block getRenderedBlock() {
+	public BlockState getRenderedState() {
-		return renderedBlock;
+		return renderedState;
 	}
 
-	public void setRenderedBlock(Block block) {
+	public void setRenderedState(BlockState state) {
-		this.renderedBlock = block;
+		this.renderedState = state;
 		markDirty();
 	}
 
 	@Override
 	public void fromTag(CompoundTag tag) {
 		super.fromTag(tag);
-		renderedBlock = Registry.BLOCK.get(new Identifier(tag.getString("Block")));
+		renderedState = BlockStateUtil.fromTag(tag);
 	}
 
 	@Override
 	public CompoundTag toTag(CompoundTag tag) {
 		super.toTag(tag);
-		tag.putString("Block", Registry.BLOCK.getId(renderedBlock).toString());
+		BlockStateUtil.toTag(tag, renderedState);
 		return tag;
 	}
 

src/main/java/io/github/cottonmc/slopetest/block/entity/render/SlopeTestRenderer.java

@@ -1,7 +1,7 @@
 package io.github.cottonmc.slopetest.block.entity.render;
 
 import com.mojang.blaze3d.platform.GlStateManager;
-import io.github.cottonmc.slopetest.SpriteSet;
+import io.github.cottonmc.slopetest.util.SpriteSet;
 import io.github.cottonmc.slopetest.block.entity.SlopeTestEntity;
 import net.fabricmc.fabric.api.client.render.ColorProviderRegistry;
 import net.minecraft.block.BlockState;
@@ -35,13 +35,13 @@ public class SlopeTestRenderer extends BlockEntityRenderer<SlopeTestEntity> {
 		Direction dir = state.get(Properties.HORIZONTAL_FACING);
 		SpriteSet sprites;
 		int color = 0xffffff;
-		if (be.getRenderedBlock() != Blocks.AIR) {
+		if (be.getRenderedState().getBlock() != Blocks.AIR) {
-			BlockState blockDefaultState = be.getRenderedBlock().getDefaultState();
+			BlockState renderedState = be.getRenderedState();
-			BakedModel model = minecraft.getBlockRenderManager().getModel(blockDefaultState);
+			BakedModel model = minecraft.getBlockRenderManager().getModel(renderedState);
 			sprites = new SpriteSet(model);
-			BlockColorProvider blockColor = ColorProviderRegistry.BLOCK.get(be.getRenderedBlock());
+			BlockColorProvider blockColor = ColorProviderRegistry.BLOCK.get(be.getRenderedState().getBlock());
 			if (blockColor != null) {
-				color = blockColor.getColor(blockDefaultState, be.getWorld(), be.getPos(), 1);
+				color = blockColor.getColor(renderedState, be.getWorld(), be.getPos(), 1);
 			}
 		} else {
 			sprites = new SpriteSet(minecraft.getSpriteAtlas().getSprite("minecraft:block/scaffolding_top"), false);

src/main/java/io/github/cottonmc/slopetest/util/BlockStateUtil.java

@@ -0,0 +1,46 @@
+package io.github.cottonmc.slopetest.util;
+
+import net.minecraft.block.Block;
+import net.minecraft.block.BlockState;
+import net.minecraft.nbt.CompoundTag;
+import net.minecraft.state.StateFactory;
+import net.minecraft.state.property.Property;
+import net.minecraft.util.Identifier;
+import net.minecraft.util.registry.Registry;
+
+import java.util.Optional;
+
+public class BlockStateUtil {
+	public static BlockState fromTag(CompoundTag tag) {
+		Block block = Registry.BLOCK.get(new Identifier(tag.getString("Block")));
+		CompoundTag properties = tag.getCompound("Properties");
+		StateFactory<Block, BlockState> factory = block.getStateFactory();
+		BlockState state = factory.getDefaultState();
+		for (String key : properties.getKeys()) {
+			Property<?> prop = factory.getProperty(key);
+			if (prop != null) parseProperty(state, prop, properties.getString(key));
+		}
+		return state;
+	}
+
+	public static CompoundTag toTag(CompoundTag tag, BlockState state) {
+		tag.putString("Block", Registry.BLOCK.getId(state.getBlock()).toString());
+		CompoundTag properties = new CompoundTag();
+		for (Property<?> prop : state.getProperties()) {
+			String value = state.get(prop).toString();
+			properties.putString(prop.getName(), value);
+		}
+		tag.put("Properties", properties);
+		return tag;
+	}
+
+	public static <T extends Comparable<T>> void parseProperty(BlockState state, Property<T> property, String value) {
+		System.out.println(value);
+		Optional<T> optional = property.getValue(value);
+		if (optional.isPresent()) {
+			System.out.println(optional.get());
+			state.with(property, optional.get());
+			System.out.println(state.get(property));
+		}
+	}
+}

src/main/java/io/github/cottonmc/slopetest/util/SpriteSet.java

@@ -1,8 +1,10 @@
-package io.github.cottonmc.slopetest;
+package io.github.cottonmc.slopetest.util;
 
+import net.minecraft.client.MinecraftClient;
 import net.minecraft.client.render.model.BakedModel;
 import net.minecraft.client.render.model.BakedQuad;
 import net.minecraft.client.texture.Sprite;
+import net.minecraft.util.Identifier;
 import net.minecraft.util.math.Direction;
 
 import java.util.HashMap;
@@ -11,10 +13,11 @@ import java.util.Map;
 import java.util.Random;
 
 public class SpriteSet {
-	Sprite global;
+	private Sprite global;
-	Map<Direction, BakedQuad> quads = new HashMap<>();
+	private Map<Direction, BakedQuad> quads;
-	boolean singleSprite = false;
+	private boolean singleSprite = false;
-	boolean globalHasColor = false;
+	private boolean globalHasColor = false;
+	public static final Sprite FALLBACK = MinecraftClient.getInstance().getSpriteAtlas().getSprite(new Identifier("minecraft:block/scaffolding_top"));
 
 	public SpriteSet(Sprite allSprite, boolean hasColor) {
 		this.global = allSprite;
@@ -24,19 +27,22 @@ public class SpriteSet {
 
 	public SpriteSet(BakedModel model) {
 		Random rand = new Random();
+		quads = new HashMap<>();
 		for (Direction dir : Direction.values()) {
 			List<BakedQuad> quads = model.getQuads(null, dir, rand);
-			this.quads.put(dir, quads.get(0));
+			if (!quads.isEmpty()) this.quads.put(dir, quads.get(0));
 		}
 	}
 
 	public Sprite getSprite(Direction dir) {
 		if (singleSprite) return global;
+		if (quads.get(dir) == null) return FALLBACK;
 		else return quads.get(dir).getSprite();
 	}
 
 	public boolean hasColor(Direction dir) {
 		if (singleSprite) return globalHasColor;
+		if (quads.get(dir) == null) return false;
 		else return quads.get(dir).hasColor();
 	}
 }