elastic. you can make a hinge from a strong strap at the back of the head, to let it tilt back, and put elastic on the sides, up near the top. they'll pull it closed while it's on your head, but allow you some wiggle room to pull it back and get your head in and out.
just make sure you've got some sort of structure of padding inside the helmet that keeps it on your head securely, you won't want it just wobbling around on your head with the elastic straps, it'll separate as you move and look off
I still believe magnets are the best way to go but you may not have the best magnets for your application l. Go check out this website and use their pullforce calc and help info to figure out what size and how many magnets and what configuration works best for you
I will say that you should be looking at the N52 designated magnets they have the strongest pullforce in the smallest size.
i was looking some option like clamps or something similar, the elastic doesn't work cuz i need to remove the whole piece to put the helmet (i cant put in the helmet is very tight to the size of my head).