what material/s should i use to create a mould and then applying pre-preg to that mould.i imagine a foam of high temperature resistance cause in the process we have to cure it in high temperatures right?any other ideas?i want the material to be as cheap as possible.

Yes, you could use the epoxy tooling board - do you need that level of accuracy though?

Why not just make a fibreglass mould? By far the most straightforward and cheapest method.