I have used several different forms over which to papier mache masks. A
milk jug (plastic) cut in half is a great form! The kids can choose if
they want the handle or not, and then build up from there using toilet
paper tubes, egg cartons, etc. This is usually when we do fantasy
masks. For African masks, take two strips of posterboard and staple them
together. They can use it as a circle or form it into a more face-like
shape. Tape it so it stays in the right shape. The kids then stuff the
circle (or whatever) with newspaper and add features with tp tubes, etc.
Papier mache over this.
I agree that this is a great way to incorporate other subjects into art.
The kids are really motivated. Last year I had my 5th graders write a
story to go with their mask. It was fiction (obviously) but it could be
written from any slant: Historical fiction, A case study of one of the
fictional people who wore the mask, how this mask was used by a fictional
tribe (rituals, etc)... The stories were really awesome and the parents
were just as excited as the kids!
Hope this helps!