I'd seen a few rack mounted pi options but couldn't get them to work as I wanted. I've simplified slightly to make printing and construction easier. The tray needs attaching to the faceplate with superglue.
I've only printed once and it seems to work ok but your results may vary.
Source is on GitHub
https://github.com/oliverclarke/rpi-rack-tray
The author hasn't provided the model origin yet.