Many thanks to @Dev_467696 for the original - I tweaked this to mount my phone via a universal AMPS/Quad Lock plate, but it should work for a wide variety of mounts:
I've done about 150 miles so far (and one fall on a green lane!) with no issues. Printed in ASA, 3 perimeters, .5mm nozzle & 30% infill. Please use outdoor rated zip-ties!
The author remixed this model.