Ummm I cant give you any authoritative advice on that,
I'd imagine sheet metal (steel).
That, then you weld it in, sand down the welds (or not) and coat it with rust preventative stuff, I've read POR15 is great, then paint it.
Do a bit of research, there's TONS of stuff out there (on this forum!) on this.
Especially: find some threads on here where people TOTALLY tear down and rebuild their cars from the chassis up-thats where a lot of rust removal stuff is covered.