I picked up a West German flecktarn camo helmet cover off of eBay the other day......... dates to the 1990's. It's got that field weathered look and a small military-made repair on the top.
It fits perfectly on the German "novelty helmet" that I'm using for the bike helmet on the BGS bicycle.
I'd like to pick up a WW2 era Swiss or German cover, but until then, this will definitely do!
Here's a shot of the helmet cover in use by the West German Army. Check out the para-cording threaded through the foliage loops to form a sort of webbing effect.