You could get “helmet adapter caps”, similar to the rubber ones you wear in swimming pools. It would be much more compact than a helmet, and could solve both th

You could get “helmet adapter caps”, similar to the rubber ones you wear in swimming pools. It would be much more compact than a helmet, and could solve both the hygiene (by virtue of being hermetic around the head) and size (by “padding” the head to some standard dimension so the helmet fits well. Then the helmet could tighten. (Disclaimer: Just an idea.not sure how practical it would be)