I'd go with papier mache. If you can find a cheap plastic "blank" mask to use as a base, all you have to do is rough-in the features you want with lightweight materials like wadded aluminum foil, masking tape, wire or cardboard, then cover with a few layers of papier mache (blend toilet paper and water in a blender, strain out most of the water, mix in some white glue, and use like modeling clay to finish the mask). Sand and paint. Blank masks can usually be found at costume rental shops or party supply stores year-round. If you don;t like working with papier mache, you can use Crayola Model Magic instead, but it's less impact-resistant.