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
I’ve added a version to use with the bumber/cover: