HEATH ROBINSON (but works) BOTTLE CARRIER
Water carrier made from a 4“ plastic drain pipe ...this holds an insulated wine bottle ”bag“ that will take a 1, 1.5, or 2 ltr plastic water bottle (2 ltr is a bit of a tight fit).
Velcro on the carrier and bottle bag stop it jumping out when on rough tracks.
I used the bottle bag on my last trip and it kept water cool for most of the day and worked well ..other than the fact I had to get off the bike and attach it to the back rack every time I stopped for a drink. I have now used the drain pipe carrier for over a year and it certainly seems a good solution for quick access to cool water.

Cut along red line