Making a mold off of bone
How about making the final project mask directly from papier-mache without all the casting: 


Cover the skull in clingfilm to protect it;
Papier-mache parts of it (if you try to do it in one go you won't be able to take the thing out, naturally);
Glue the parts together;
Alter it as you desire - cut some bits here, add some bits there, sand it;
Paint it, if needed


Paper-mache masks are light, resilient (as long as they are not too thin) and you can waterproof them with a spray if you wish.