Seriously, though, instead of trying to affix a bulky water bottle inside something that could get entangled or lost during bailout, what about using a version of those emergency water packets that lay pretty flat? e.g. 4oz with a 5 year shelf-life: https://www.moreprepared.com/sos-eme...ng-water-pouch
You could place 1 or more in various pockets, inside your shirt or some other clever location? I'll ask my rigger for his input.