i have never tried but I probably wouldnt recommend using polyurethane on the inside. I would think it would need to be decently aired out before it would even start to dry. I can only imagine the heavy smell that would also need to air out without having that suffocating smell while wearing it. Unless you applied it to the foam, let it dry, and then inserted it but not sure.
The mask i got from inkmonster was secure by the foam alone. on a different mask it had a with really big head had a similar issue with the dome. It was a 1/1 size homer simpson head mask. I used foam and masking tape to secure the round shape at the top and sides. His head had a semi-permanent dent in it when it was packaged, but didnt have any issues afterwards.
I havent attached my mandibles to my mask yet, but I dont think they would be an issue unless the skin around the mouth was too thin. Maybe some one can share some light on this. Always interested to learn something new.