I've seen this done before, I think I have a pic somewhere too that I can post if I can find it. Most people form it in three different parts, much like trooper helmets. The dome, face, and back of the helmet are all seperate pieces and you connect them together. Then add resin pieces on later. It would also probably be helpful to do a coat of fiberglass inside to give it some strength.
It can be done, but a lot of people just save for a fiberglass helmet
-=QuinN!