It's a remix from yazgazan on thingiverse:
https://www.thingiverse.com/thing:2085090
I edited the mount part, so I could screw it on my electricity distribution box. Hole to Hole is now 25mm instead of 26mm, and I used flat head screws.
Update: Added 3mm distance between RPi and backplate
The author hasn't provided the model origin yet.