Aha! Doc Dailey I think you may have inspired me, with your problem's arising with Spartan helmet's being too small to be simply removed on and off. How about if we say cut out 2 small panel's that would allow for bigger access to fit the helmet, while installing Rare Earth Magnets (Or fastener or your choice Magnet's just happen to snap in place and come off easy.) so that the panels come on and off easier.
Sounds a little crude but if you have a visual mind like I do you can picture this all working together seamlessly, Put the Helmet on your head, quickly put the two small side panel's too seal the deal. Of course you will have to remove the panels to say remove the helmet but the magnet's should make that quick and easy,
Another option might be a simple hinging mechanism to open it up a little more maybe..? I'm not entirely sure, TBH I just read a post in this thread and got this idea just now