Hello Stan Winston School of Character Arts! I think I've found another way to make the under-skulls for your animatronic creatures. I'm sure that by now, everybody who's ever done special effects (as a hobby, or otherwise) has heard of Pepakura, the software that enables you to print paper-craft models. I've seen people use it to make Sci-fi armor out of EVA foam or fiberglass, but I've just figured out how to use it to create animatronic under-skulls. You can print-out a Pepakura animal mask like this one
and fiberglass it, or just fill the internal cavity of the model with plaster of Paris to make a Vaccuform mold.