I have been keeping deionised water in a wine cask in the beer fridge for this in Brisbane. The most important (to my mind) is the first stages of process and for the rinse I gradually let it rise to the tap water temp. A bucket of iced water (no special treatment, perhaps some Calgon, but cooled w ice cubes) slowly rises to ambient which will do no special harm is for rinsing. It's a challenge indeed. The bucket water becomes the rinse water.
It can be really hard to keep the tank cool but I let it float in the bucket especially if I'm using a stand developer like, say, a Beutler. If you don't watch out (as I'm sure you know) the temp in the tank can soar during a 30 or more development time.
The cask water is usually enough for 1 film.