Great Stuff is dreadful at best. It works great for what its made for.
Flex foam, from Smooth-on should work good.
FlexFoam-iT! ® Series Castable Flexible Urethane Foam | Smooth-On, Inc. It is practically the same thing as Nerf foam.
I have cast it in my latex skull molds before, without messing up the latex. I would however, recommend lining the inside with a trash bag. So basically you pour the foam in the garbage bag. For a support shell, you could stuff the mask with paper getting desired shape, then do paper mache or plaster around it. A paper mache or plaster support shell should work just fine.
IT is somewhat pricy, but terribly more than great stuff. It does help to have the recommended release agent. The skin of the foam will be effected by the release agent.