I use Office Depot clear plastic file boxes. They go for $10 on perpetual sale. Then I stack them on a chrome wire NSF shelving unit from Costco (4' wide 6' high x 18" deep) which I bought a bunch of for $73 each. I use them for all kinds of storage, including helmets. A shelving unit holds up to 12 boxes and is rated for 600 pounds.
I like the shelving units because they're clean, strong, they roll on casters, and I've never seen anything for less than $73 that wasn't particle board or plastic.
I like the file boxes which are polyproplyene because they keep out dust, moisture and rodents (unlike cardboard boxes). They are the perfect size for helmets (one helmet per box). They keep the helmets from dust, scratches, and they prevent some falls because they will not roll like the helmet itself.
It is a bit too costly in materials and space for basic bicycle/skate type helmets, but if your helmets are many hundreds of dollars it is worth it.