I'd like to make a large number of cheap skulls, to be used like the debris in the Terminator 2 opening. Making the mold is easy, I've done those before, but what would you suggest for material to fill the mold (or surround the solid one with)? Cheap is more important than anything. It doesn't have to look very good, as it would barely be seen anyway. And since I don't want a lot of molds but do need a lot of skulls curing time is a factor.