I use my Peak Design duffel as gym bag and needed a water bottle holder. I found a great solution

I wanted to find a way to attach it so it wouldn't flop around loosely. Most of the molle water bottle holder mounting points were too narrow. Eventually I found one made for a bike with a bunch of mounting points. It came with hook and loop straps but they wouldn't get tight enough and led to flopping. So I used zip ties. They worked wonderfully.