You spend tons of money off clay and sculpt the helmet, make a plaster mold, make a mold of the mold so you get a replica of the clay piece, vacume form the final plaster piece with hot soft plastic.
I think the best way to do the helmet would be to do it in 3 or 4 molds, such as the top, the sides, then the back, it's impossible to vac-form the helmet as one peice with 1 piece of plastic.