You might be able to find one of those helmet covers from back in the day when helmets were uncovered, hard white foam that you put a fabric cover over. I think I still have my old Salsa Bikes red lace panty helmet cover. It has lace down the middle that covers the vents and just plain stretchy fabric on the sides. If you're interested, I'll see if I can dig it up (though I just tried to declutter things and it may have gone out with the clutter).