I had the idea for a custom PC chassis before, but I didn't have the tools to fabricate one until I got a 3D printer. The first order of business is to verify that my 3D printer can hit the necessary dimension requirements to print a functional PC chassis.
I started with a small Mini-ITX form factor motherboard I had on hand. The footprint of the motherboard is 17cm square, conveniently fitting within the 20cm square print area of my 3D printer. By printing a small functional enclosure for the Mini-ITX board (including the metal back plate for the ports) I gained confidence that it is possible to 3D print components for a custom PC chassis.
This small test only enclosed the motherboard and none of the other parts needed for a full PC. A fact clearly visible in the picture as exposed tangled nest of wires.
If such an enclosure would be useful to you, click the "Mini ITX enclosure" link in this project to view the OnShape public document. From the document you can export the objects to STL and print on your own 3D printer.