Cardboard

Reading tutorial's on thedentedhelmet.com a lot of people build helmet with a cardboard
You mean the normal cardboard? This from the carton boxe?
http://www.allegro.pl/item749300538_...5zl_szt50.html or http://www.allegro.pl/item746158408_...na_fala_b.html

Then just cover with gypsum?

Thanks

The stuff you want is Mat Board

You use it (normally) in picture frames, its very rigid but easy to cut, and easy to shape with water.

Search up AntMan's cardboard buildup threads.

Good luck!

EDIT: http://www.thedentedhelmet.com/f23/first-post-first-wof-bukcet-build-cardboard-23159/ here you are, because I'm nice