Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:3906331.
This is a mountable mount for a Micro ATX or Mini ITX motherboard. Not only can you mount your motherboard and PSU on this, but there are also four holes on it to allow you to mount the whole thing to whatever you want with screws; or, you could just secure it to your work bench to keep it from moving around when you are testing out devices.
This is printed in several parts to allow for printing on printers that aren't massive. You could even fit it on a 6"x6" build area if you trim a bit off of the ends of the tabs, which shouldn't affect functionality.
It is designed to be press fit for PLA on my printer, YMMV with your printer's tolerances or the material you use.