Water bottle cages for bicycles aren’t expensive, but they’re also not adjustable if you don’t want to carry around the standard-size water bottle. Instructables user marsrover has a guide to making one for yourself out of a coat hanger.
The process itself is about as simple as you imagine. Fold the coat hanger to the desired shape, tape off the ends, and then mount it to the bike with screws and washers. The benefit is that you can adjust it as you go along and make it work for whatever size water bottle (or anything else) you use. Hit up Instructables for a full guide.
Coat hanger water bottle bike holder [Instructables]