Drywall compound....of course. This seems like such a better idea than glued paper (machee?) and such. Maybe because I'm just used to working with it? I've never seen it recommended before, but it looks like it would be way easier to shape. Thanks for the tip.
Sculptimold or paper machee is better because you can shape it into forms that the drywall compound will never hold. Drywall compound also shrinks and cracks a lot.