Building first W.O.F cardboard and discovered this

Ive used all difrent types off helmets for my domes ive been lucky and found a few that were the right cercumfrence they can make great domes just remember to adjust the mesuerments to the size of the lid to get the right over all preportions, the one im working on now i made out off the dome from a hasbro boba helm (the only part worth saving imho) and handy pail plastic paint pots fer the rest. they allready have the curved shape so you dont need to heat and bend like pvc,

Only found this out yesterday put it up soon as i had something to share The shape and size of the helmet is almost perfect and with straps thinking it could make a very strong, comfortable helmet. Considering thats what it is

lots of people use real helmets for the dome, usually kids skateboard helmets like your kids.
other have gone as far as to use DoT approved biker helmets as the base, I have even seen construction helmets being used.

all depends on your wallet, skills, needs and wants.