Another hot day with the lawn. Rather than stop for a drink of water, just make a holder for the bottle, so the mowing can be completed quicker!
A multitude of options in this OpenSCAD script. Sadly, not set up for multi-part creation yet; maybe a future rev.
Of primary note is the "bottle_diam" and "mount_diam" parameters; these control the size of the bottle and handlebar, respectively. As coded, the mounting holes are for M3(x16) bolts.
If your bottle has a bigger top, you may be able to print just a "top", letting gravity do the job. Otherwise, a "bottom" will be necessary to counteract gravity...