Options I can see (many already discussed)
Handle bar cage - Evans sell these
Seat post cage, they actually sit quite high up so are unlikely to bottom on a dropper post but with the dropper down you might find you get a bottle in the nut sack!
You could try a cable tie mount but underneath the top tube with some car trim tape (strong double sided foam padded tape) to help keep in in place, by having it underneath gravity will help to stop it rolling as well.
Change bike, get the focus - an expensive option but if has a place for a bottle cage
Potentially you could fabricate a cable tie round the batter job, I'm assuming it's flat topped but you need to ensure your not putting any pressure on the mount on the connectors - I'd leave this option well alone, keeping 500ml of water next to the batter is asking for trouble, as it modding the mounting.
Or just wear a camel back, you can get ones that go inside bigger back packs as well, I have a. Berghous one, it's got netting for a helmet on the front, a section for a baladder inside and room for a large D-Lock, 2m thick cable lock, pump, tubes and a host of other stuff but the bag itself is quite small.
Lastly, plan lots of pub stops on your ride [emoji12]
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