I recently finished some masks for  theater production, and one of my concerns was fit for various actors as well as their ability to dance vigorously and perform aerial stunts. I think I found a solution that may benefit some here, as well as many who come here seeking answers around Halloween.
I had considered building the masks around a hard had, as many do, but that would make the masks too big and bulky. While fiddling with the hard hat, I realized that the harness inside is made to be removed easily  and replaced. The mechanisms vary, but basically there are small tabs that fit into slots on the hard hat. I simply used small bolts and nuts to attach these tabs to the interior of the mask. The harness keeps the helmet and any protruding nuts (I did use the shortest that would work) away from the actors head. It also allows for air flow, reducing overheating that would occur with layers of foam.
The harnesses are adjustable to fit a normal range of head sizes from about sizes 6 1/2 to 8 1/4. Extra small and extra large versions can be purchased also.They also come in various degrees of plushness, with terry cloth and sometimes chamois sweat bands. The price is nice too. They average about $5 US, depending on options, when ordered through a industrial supply company. I think the ones I ended up ordering were about 3.50 each, and they were very nice as far as fit and comfort.
One suggestion though. Do not order the type with the ratchet knob for adjusting sized. While it is the easiest way to adjust fit,the knob juts out from the back of the harness and may not fit inside the mask. The sliding adjusters take up no more room and only require a little more effort to size.
In the end, they worked beautifully. The dancers were comfortable as possible and were able to leap and flip with no movement of the masks. I think this method would be good for helmets and even large headpieces. hats and crowns.
Hope this is helpful to some out there. I know it was an epiphany for me
Joey