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Depends on the helmet. Some larger ones are tight in mine. My older 45L Junior topcase works much better for that.Actually it's been a while but I think the 40L Junior sidecases were the same (some helmets won't fit).
The 55L Junior topcase that replaced the 45L will take two helmets, even Size-L modulars.
Tom, I liked those eBay boxes you posted the link for! I'll have to contact the guy, it looks like he may do fabrication for mounts on various bikes. Heckuva deal at $90 (a box I'm assuming).
Careful with some of those Mutazu or whatever Ebay bags. We had a bud who put a set on a W650, they were absolutely pieces of thin plastic garbage (besides being about as ugly as possible).