Reading tutorial's on thedentedhelmet.com a lot of people build helmet with a cardboard :confused
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?
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.
EDIT: http://www.thedentedhelmet.com/showthread.php?t=23159 here you are, because I'm nice :D