A lot of the kits you see use fast curing resins and plastic beads to bulk it out as you sit on it during the cure.  Doing something similar as Hanaldo says would get you the shape you can use to make the pattern.

If you wanted the full technical look, you could even replicate the extraction straps and bits and bobs to make it look more real.

This PDF shows the single seater extractable seat rules which has some good pictures and information:
 


Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Super easy to do. Just fill a garbage bag with 2 pack expanding polyurethane foam and sit in it while it cures. 

That's how I made the mould for this 1968 Brabham seat:

so you are looking into something that looks like the carbon hammock bath?

You can buy sheets of the same mouldphorm material that easy composites sell, and use heat to formit to a shape and then a mould. but otherwise you will have to design something and have it cnc machined from a suitable epoxy tooling block.

Do you want to do a proper carbon f1 seat ? Or a foam splash moulded to your shape to drop in a tub

Hey everyone! I am planning to do a F1 seat, 
i was wondering if there is such thing as a foam wich gets hard after a while so that you can use it as a mould? 
Or how would you build the mold? 


thanks!