A pretty simple piece. Dimensioned for a socket of a 2002 Volvo, but should work almost everywhere. 

On the outside insert a M4 nut and you're ready to go attaching GoPro's or anything else you make compatible with the GoPro standard (phones, GPSs, etc.)

I printed it standing up on the plug (as in the 3mf) to get a clean connection to the car, this does make the fingers on top easier to break, but fine in my experience. The 4mm hole for the screw is also shaped to allow for this orientation with no issue