Ya i think me and link are confused about the same thing? Are you making a mold from the dirtbike helmet? Or are you building on top of the helmet?
I wouldn't sugest making a mold of the helmet. if you want to carve it I would measure A box the sizes of the helmet then add an inch on every side.
build a cardboard box hot glue all the seams and brace the sides. then I would mix Plaster and pour yourself a large block that you still be able to carve into and will remain workable for up to two weeks if you keep in in a cool shaded area. :lindsey:
And that you can make a mold from. and slush cast in liquid plastic or resin or something hard and strong like that. :eyepatch: