This is a case for Sven Petersen's Diag64cart, an open diagnostics cartridge for Commodore C64 and C128 computers.
The main work was done by Laurence Gonsalves who published the OpenSCAD source code of a customizable C64 cartridge (CC BY-SA). I shrunk the cartridge to its minimal size, added openings for the button, LED and switches, and removed the label area. Thanks to Sven's excellent documentation, it was easy to find the right positions for the case openings.
This case is designed for the Diag64cart Rev 2 or Rev 3 (and maybe even later revisions), and is tested with Rev 3. For older revisions, use the case top without the LED. The source code with my changes is available in this project, too.
The other cases of the harness set can be found at Sven Petersen's repository.
The original design was intended to be printed in a standing orientation, with the opening on the print bed. I preferred to print it with the top/bottom side on the bed, which works amazingly well even without supports.
The “C64 Diagnostics” sticker on the photo is sold by Badgeman. He is also selling stickers for the other parts of the harness, but the one on the photo were made by myself.
The author remixed this model.