38-40 no problem fitting.
I bought 3 waist packs, I've found a place near me that sells them for under $20. actually,
the most expensive one was $18.
if you're carrying heavy items like bottles that may shift and sag (like you posted) find one with a wide rigid back belt. like a weightlifters belt.
they should have side cinch straps to keep the stuff close and compact.
since I've spent money on 3 packs, they're used for different purposes. small for just one bottle
camera, mp3. medium and large for more than one bottle, cable lock, lunch, spare batteries and misc
sports stuff.