New Inventory design/functionality

I was wondering if someone could point me in the right direction.

is there a way to hook/remove/create your own hand selector? im talking about when in game you scroll using your mouse scroll wheel to choose between your keys, physgun etc. Could you hide the default grid view which shows your inventory and make your own design/hook?

Look how TTT did it for example. Yes, it is very much possible.

Can you point me in the right direction with some links/references or code sample?