Thanks to @Volker I just had the opportunity to test my official Fairphone 4 flip case with an FP5. (You can find the test of the case with the FP4 in this thread. Here’s what we found:
In general the FP4 sits in the case pretty well and tight. At a first glance the FP5 definitely seems reasonably protected by the case:
All buttons on the right side, the microphone at the top, as well as the openings on the bottom (loudspeaker, USB C port, microphone) remain accessible:
The microphone on the left is obviously covered, as the FP5 microphone is in a different position. The FP4 microphone hole is useless, but also not counterproductive. This issue could probbly be fixed by drilling a little additional hole in the position of the FP5 mic:
Another issue is that the camera is not a perfect fit. There’s a little gap at the top-left corner and not enogh room at the bottom-right edge. I suppose this could also be easily fixed by filing or cutting away a little bit of the diagonal edge:
Also note that the curvature of the FP5 corners is a little bit different than that of the FP4, so there is a small gap between each of the phone’s cornera and the case as well. Overall the phone still sits pretty tightly in the case, with the exception of the right upper edge, which tends to slide outwards a bit and can be bent away with some force. This is partially due to the slight camera misfit. Overall the other three corners are still enough to hold the phone in place tightly, making it reasonably hard to remove the phone from the case.
Also note that since the FP5 is a bit slimmer than the FP4, there is an additional edge reaching above the display, which might actually be considered an advantage by some:
Overall I would consider the case usable, if one is willing to make the mentioned corrections for the camera- and microphone-misfit, but a perfect-fit case for the FP5 would obviously be better.
(Also note that I forgot to actually insert a USB-C cable while the FP5 was in the case, so you’ll have to judge that fit by the photos I made.)