Slim hydration backpack
 

I'm looking for a slim hydration rucksack, the slimmer the better. Since we travel two-up on a XT660Z Ténéré there isn't much space between my wife and me. I've tried my old hydration backpack today, about 17cm (7") deep, and that was way too much. She felt jammed between the topcase behind her and my backpack.

Any hints what I could get in the 5-7cm range?