I'm an add-on developer, and I'm curious how you made the gravestones. I understand that they are player entities with custom models, but what I am unsure about is how the hitbox works. The hitbox acts as a solid block, and is not a full block (so it can't be barriers) and I don't know how they would change the hitbox type (as in normal or Shulker) of a specific entity without making a new type.
