Now available on Etsy complete with free shipping and embedded threaded inserts.

This accessory rail mounts to the back of your VESA-compatible monitor or arm.

It has place for two ¼" threaded inserts and one GoPro accessory mount.
I used ¼" brass threaded inserts from CNC Kitchen but have not seen them for a while now on Amazon. You can probably modify to print in place thread.

I used it to hold my vintage Apple camera and (more functional) old Android phone to use as excellent webcam with Camo app. I've seen people use it to mount DSLRs, microphones, lighting, and even a Mac Mini to the back of their monitor.

Printed with 0.2mm layer height in PLA with 0.5mm nozzle.