Hello! Over the years ive collected quite a few FP4 spare batteries and it was kinda annoying to keep them charged using the phone itself, so ive created external 3d-pritable case with slits, so you can connect battery to external charger or even power other electronics.
Please note that Fairphone 4 uses Li-HV batteries that have different voltages than typical Li-Ion.
I shared the STL file on thingiverse and design on tinkercad, feel free to use.