Source code (designed in SolveSpace): https://github.com/nazar-pc/matx-open-test-bench

It only has motherboard tray and support for 4 expansion cards. No I/O shield or anything else, has cut-out on the back for easier cooler and back SSD access (if any).

I was printing it with 1mm nozzle, 0.5m layer height and 30% infill in PLA on Creality CR-5 Pro H.

Motherboard can be secured with M3 screws and expansion cards with 6-32 screws like most standard cases. They will tap right in, no need for inserts or anything.

STL is shared under Public Domain, if you want to modify sources though, check GitHub for license on source files.