Recently bought my daughter a bike without any water bottle mounts. This clamps onto the handlebars and accepts a standard size water bottle cage.
I've printed this out but haven't tried it yet so if you do, let me know how it works out!
This mount requires 3 M5 nuts, 1 M3 nut, and 1 20mm M3 screw. Assembly should be self-explanatory. The M5 nuts are the same size as the braze-on water bottle bosses so a cage should attach using the same screws you would use on any other mount.