Hello, I am still new to T.D.H, all I can say is: I'm glad I found you guys!
I am halfway through a scratch build of a Boba Helmet, which I'll be painting up in a kinda ESB scheme with my own touches. (photos to come soon)
I just wanted to share a tip with you guys, I just hope it proves useful.
I work in an art gallery/picture framing shop and decided to use Mountboard (the cardboard borders you see around your picture's image in the frame) as it's very robust and bends easily (spray mist to help it hold it's shape as it dries). You can buy this type of board from hobby shops and art supply places like Hobbycraft or Granthams for a few £'s per sheet, you'll need several so it's not exactly dirt cheap but the helmet I have built with mountboard is very sturdy and strong, no doubt a layer of fibreglass inside the helmet will boost that strength but for those who won't be able to use fibreglass mountboard gives a nice strong option.
I hope this is useful for anyone wanting to know what material to use.
Also, Antman's excellent tutorial will help you get your head around the templates you'll be using.