First, make the foundation. It should be even and supportive. Then you create the infrastructure using crossing metal bars. the should make an X and if enough material is handy a + over it.
What you need is tin foil string then some sticks .1. you tie the sticks together like a raft with the string then when you get it the size and shape you want then cover the entire thing with tin...
they're are home remedies to making prosthetics. but it'd be best to buy the actual things needed. for most prosthetics you need to use things like spirit gum, liquid latex etc. There is youtube...