I've tried a lot of ways to get my water bottles fastened on the floor of the cockpit to make use of the available space, and I think I have found the perfect system. These adhesive mounts from 3M are very strong to the point that you need a scrapping knife to remove and after a month of use seem to be completely waterproof. I've combined them with a bungee chord which I use to secure the bottles and they stay in place whenever I roll or do a wet exit.
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