I’ve made a Fairphone 4 bike mount for 4 Quick Fix System adapter.
I’ve used the Simple 4 Quick Fix Worldwide Mount Plate made by @RoboMagus (thank you!) and fused it into my bike mount.
I bought this cheap QuickMOUNT from amazon for my bike.
The files are on printables.com
https://www.printables.com/de/model/439296-fairphone-4-bike-mount-for-4-quick-fix-system
Edit:
I’ve added a version to use with the bumber/cover: