I wrap the base with thin foil, sculpt over that and once finished put the whole into the freezer. Sculpey will get slighly harder so you can unwrap the base putting it out without ruining the sculpt and then bake JUST the sculpey (otherwise tthe pastic base will melt but most likely will be warped at its edges).
Once baked, pin the piede on your plastic base. Another thing you can try is to make a mould of your plastic bases, then cast sculpey bases. Once sculpey bases are baked, you can use more sculpey over them and then bake again at no risk.
hope this helps.
:beer: