Custom model problems

Hey, I've started a mod and added a block with a custom model, but when It's in my inventory slot or I hold it in my hand, it has only bottom pixels shown (Like if you see its side in 2D).

So I've thought about 2 solutions:

1) Get it to render properly

2) Create a 2D item for the block

I've thought of the number 2 but I don't know how do I add this item. 

Imagine a cake, the handheld item is different than the actual block.

Can anyone please help me?