one option is a small hydration pack. find clearance for $25 or less... just remove the water bladder. super comfortable on trails. and doesn't move around on the body-if it has a front chest or waist strap. just roll up your jacket. I carry my jacket inside along with a 2L badder.
Bass Pro Shop is just one example for $15.
https://www.basspro.com/shop/en/bass...hydration-pack
of course you can bungie your jacket to your toptube...just be sure to secure the arms!! The last thing you want is having the arms get caught in your chain or wheel.
EDIT: I use an Osprey Syncro 10 with 10L volume, similar to this
https://www.osprey.com/syncro-5-sync...50#color=Black