Designed to fit a Victorinox boning knife, model number 5.6603.15 with the Fibrox black handle.
If you want the TinkerCad link, let me know. Prints with the flat side down, no supports. 0.2mm layer height works fine, but I ended up going 0.15mm to make the logo a little cleaner. To fit on a Prusa Mini, just rotate 45°, or 22.5° for multiple. I've used PLA so far and it's worked great.
Warning: I haven't tested these for more than a few days, so I don't know if after time the blade would cut through. Use at your own risk, unsheath with the blade's edge pointed away from your hand.
https://www.tinkercad.com/things/4peKAvPgQBG-victorinox-6-semi-flex-boning-knife-sheath
The author remixed this model.
A riff off of @EMAXX620's great work, I just elongated the design to fit the Fibrox handle, added a logo to both sides that helps with gripping, and trimmed to a 22.5° angle to match the handle.