A colleague of mine made a custom fit phantom mask by doing th following:
Step 1. Put on a swim cap and cover your face with vaseline (petroleum jelly), then get a friend to cover your face with strips of plaster bandage to take a cast of your face. Ensure that the cast comes back to your ears and use a piece of straw in the mouth to breathe through. (Alternatively don't put the plaster bandages over the nostrils)
Step 2. Once the plaster bandages have dried, carefully remove from your face. Fill any holes and cover the inside with a thin layer of petroleum jelly and place this in a sand box or similar to hold it's shape. Mix up plaster of paris and pour into the mould.
Step 3. Once the plaster has dried remove it from the mould. (You may have to tear the plaster bandages.) You should now have a cast of your face to work on.
Step 4. Cover the cast of your face with a thin layer of vaseline and apply your papier mache directly onto the cast. Apply at least 3 layers, blotting paper is ideal and use watered down pva glue. Once dry this will fit exactly onto your face.
Hope this helps
AlanC