“Wove’s bottle retention loop is the solution to the widespread issue of ejected bottles, especially when using behind-the-saddle configurations. The disappearance of your hydration or nutrition can be more than just an inconvenience, which is why Wove's Bottle Retention Loops are designed to work with any two-bolt cages, to hold any bottle securely in place.
Simply attach the metal tab of the loop to the top bolt of a bottle cage, and use the pull tab to hook the loop around the nozzle of a bottle to keep bottles of any shape secure in any cage”
Wove 
Cage sold separately