Entities with a rendered block in a model part?? like minecarts and sulfur cubes

I am trying to make a block cannon where you can put blocks inside and you can see the block sticking out. But there is no way to make a part of the model a rendered block using a BlockState nbt, like minecarts. How do I make a certain part of my model have a rendered block using an nbt. Or have my custom model support armor, so i can put a block as an armor peice, like sulfur cubes. Basically like /item replace @p armor.body with minecraft:block. If you know how to do it the armor way, please tell me how to make your custom entity support armor so i can do it. :) thx for your help :) :) :) :) :) :) :D :D :))