I took the original cable bezel out of my Model M. Measured it and drafted up a new adapter that fits in the original location and has a mounting tray for a Pro Micro. The only non-reversible modification made was snipping the cable. The cable wasn't in great condition so I wasn't overly concerned with that. If it was, I could have found something else to use to connect to the original controller and avoided the damage.
Everything fit pretty well on my first attempt. Looks like I didn't screw up any of my measurements by anything significant.
Maybe I should have looked if there was a beige color to get this 3D printed in. Oh well, it's not in a noticeable location. I left the hole for the USB header a bit bigger then needed just to be safe. The cable covers that up pretty well.
The complete Model M 1395665 running the Soarer firmware. I had to adjust the configuration to make up for some of the odd key placement on this terminal model. I also swapped Ctrl/Caps and left Alt for Win key.
Sketchup | STL | Shapeways
Here are the Sketchup and exported STL models for download and a link to Shapeways where I ordered mine from.
Discuss on reddit!