I really liked ThinkAndPrint's RPi Tower rack as it's one of the few contained clusters with a fan port that would fit on the Monoprice Select Mini. One thing I wanted to add was the ability to attach a front bezel to effectively lock the pi's in place. I used Tinkercad to expand the corners and add M3 sized screw holes.

After I was able to print a good unit, I also noticed that the raspberry pi port-side vent was very tight and required scraping the insert and remove the raspberry pis. I then added a few millimeters of space at the insertion point to allow the board to be inserted more smoothly.