Large framing nails pounded into the wall.
I, like everyone else, saw all these elaborate setups using 6" diameter PVC pipe and such when I first got started, and thought it was some big deal to keep the belts from getting screwed up. Eventually, because I'm pretty lazy, I ended up hanging them on nails, and have NEVER had an issue.
Personally I believe it's one of those things, like perfect belt tracking (this is an extension obviously, I assume people believe their belts track badly because of this or that), that people fixate on, convinced that it's affecting the results of their grinding. If you can grind well, it *does not matter* if a belt has minor wobble, however regardless, I've seen zero evidence myself that how you hang a belt, affects it's tracking at all. If I ever have a 3m or norton, etc, belt that *jumps* all over the platen, half an inch of wobble or more, I simply return it to pops for a replacement, that's just a QC issue. 1/8" or so wobble doesn't affect anything, it's repeatable and consistently wobbling to X location, as long as your grinder isn't screwed up.
In fairness, I don't buy junk quality cheap belts ever. So maybe it's different with the cheapo special belts.
Humidity however does affect J-weight/flex belts, and can be really annoying when they get all curled up. Only way I've found to fix this is to store them stretched, or rolled up.
Really fine grit J-weight belts are prone to contamination anyway (some piece of stray grit from other belts, causing some stray course scratches), so I store my 800's and 1200's rolled up like they come from the suppliers, in a drawer anyway, like my loose buffs for fine compounds.