Sculpey is a "bake to harden" clay that I use all the time to make stuff for my games. I've been using it for years and years. You can frequently get a big box of the stuff for pretty cheap and it never really dries out and it lasts and lasts.
I was at the dollar store and I saw these "ice cube trays."
It occurred to me that you could press Sculpey into these and make cool skull castings. Here's one I made --
Here's one with a sense of scale --
I think they would paint up pretty nice and they would make nice additions to a 3D dungeon environment, perhaps on an evil altar or as a trap or trick...