I'm continuing (kinda) my cape topic. After discussion I've decided to make backpack instead of capes, I have backpack model that was created in blockbench and it's exported as .java class. I have two problems:

1. How to implement this model.

2. How to make void or function to render this model on player's back (also I want to make it render on players specified by UUID)

There's raw .java backpack model file (it's not in in my files yet):

// Made with Blockbench 3.8.3
// Exported for Minecraft version 1.15 - 1.16
// Paste this class into your mod and generate all required imports


public class backpack_dev extends EntityModel<Entity> {
	private final ModelRenderer bb_main;

	public backpack_dev() {
		textureWidth = 32;
		textureHeight = 32;

		bb_main = new ModelRenderer(this);
		bb_main.setRotationPoint(0.0F, 24.0F, 0.0F);
		bb_main.setTextureOffset(0, 0).addBox(-4.0F, -10.0F, -2.0F, 8.0F, 10.0F, 3.0F, 0.0F, false);
		bb_main.setTextureOffset(0, 13).addBox(-3.0F, -9.0F, 1.0F, 6.0F, 9.0F, 1.0F, 0.0F, false);
		bb_main.setTextureOffset(0, 23).addBox(-2.0F, -5.0F, 2.0F, 4.0F, 5.0F, 1.0F, 0.0F, false);
		bb_main.setTextureOffset(0, 30).addBox(-1.0F, -6.0F, 2.0F, 2.0F, 1.0F, 1.0F, 0.0F, false);
	}

	@Override
	public void setRotationAngles(Entity entity, float limbSwing, float limbSwingAmount, float ageInTicks, float netHeadYaw, float headPitch){
		//previously the render function, render code was moved to a method below
	}

	@Override
	public void render(MatrixStack matrixStack, IVertexBuilder buffer, int packedLight, int packedOverlay, float red, float green, float blue, float alpha){
		bb_main.render(matrixStack, buffer, packedLight, packedOverlay);
	}

	public void setRotationAngle(ModelRenderer modelRenderer, float x, float y, float z) {
		modelRenderer.rotateAngleX = x;
		modelRenderer.rotateAngleY = y;
		modelRenderer.rotateAngleZ = z;
	}
}

Also I don't know why, but I get error when I paste this code into class.

Any help would be appreciated.

Does it need to be a java model? The same could be accomplished with a json one.

I mean I want this model to be rendered always like in capes. If your UUID matches UUID in list you automatically have backpack rendered on your back.