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?

My 'chute has a sheepskin Velcro-ed on which essentially forms a large bin. You can toss in several water packets w/o affecting comfort since they're quite thin (125ml vol). They don't shift or fall out at any attitude. If place near the lumbar region, they can provide gentle support. I've also made an insert from a T-shirt with 6 pockets which the water packets and other small emergency items (e.g., space blanket) can be placed. This is then inserted behind the sheepskin. In addition to the sterile water (which can be used to irrigate small wounds or even chilled to cool your back) I also carry an old, out of contract, thin flip-phone that can be used to call 911. This is not a lot of extra effort, and once the items are in place there's no need to think about it.