Clay or Foam?

[T-Type]

Hey all. I'm looking at adding some custom pieces to a urethane body kit and need to work off the original kit to make the new additions. Looking for recommendations as to what would be easier to work with....the sculpting clay or foam? Keep in mind i'll need to make the new bits out of fiberglass.

Wondering if the clay would be easier to shape or foam. If I use foam will the resin melt it? Is there a specific name for the clay and where is the best place to get it? If I use the foam, what type should I look for? Closed cell of some sort? Any other tips or tricks would be appreciated as well. Thanks in advance.