There is a market in offshore, aerial work, flight training and military for helmets that are adjustable to fit most sizes of heads.
Probably would need a range of say 5 sizes?
Each helmet with adjustable strap at the base of the skull and a lever something like a series of 5 ski boot straps, that pulls the inner foam so it expands and conforms to the head.
In my view there as many casual flyers who get a thrill from wearing a helmet as those who are turned off.
Adjustable ski boots that can fit all and sundry is a pointer that it can be done for head gear too.
Mickjoebill