I'll second the apoxie sculpt. Two-part self-hardening resin that is easily sanded when cured. You can refine it with water on your finger or sculpting tool - wear gloves and use a disposable sculpting tool as the residue from the watered apoxie sculpt will harden on it if you don't wipe regularly.
It can be applied thinner than super sculpey and don't risk cracking when cured. A much better option, and basically sculpts like super sculpey.
However, if using sculpey - if you just heat the surface of it to make it rigid, you can mold it without fully curing it and peel it off afterwards and use it as filler in another sculpt. You could even freeze it and mold it then, as it becomes denser when cooled like that. At least that's what I've heard about Sculpey. But as with everything: test it first on something unrelated, but with similar qualities to your project - in this case on a spare piece of the foam you are using.
