I also collect WW2 helmets. I have a few of these in display boxes used for NFL/CFL football helmets. They look great with a mirror on the back. Unfortunately (to my surprise), Pickelhaubes do not fit given their height unless you let them sit on their brims....not a good idea. They work with WW2 Stahlhelm, but NOT the haubes. Steer clear of these types of display cases/boxes. Happy hunting.
Rob