[BWheel]

The only other option I would look at is to either build a mold, or cut the seat from a soft wood, such as balsa.

Now, the balsa option probably doesn't sound too exciting, but what you could do is cut it as you want it to look, and then either get a latex mold that you can pour other seats (not that useful unless you want the same seat for many cars), or you could varnish the seat, which will give it some good strength, and then sand it (a good few layers of varnish). This will then also give you a good base to paint onto.

Other options would be a plasticard base, with the basic shape, which you then build up with your epoxy. Similar to what you were talking about yourself, however, your basic shape is simpler to start from.

Good luck!
B