This is a customizable case for an X-Acto knife. To customize login to Thingiverse and click the Open In Customizer button. The default dimensions are for an X-Acto #2 knife as I used on my own printer.

The case as at one and a slot to protect the blade and it should be slightly longer than the blade to make sure the tip of the blade is not damage by the end wall of the case.

If you want to secure case in the close position use a rubber band though I just rely on the friction between the base and the cover (but keep the knife in a drawer, not in my pants pocket).

I printed it with PLA, 0.2mm layers, 0.4 mm nozzle, 200C, 70C PEI bed and brim to prevent curling. At the end of the print leave it to cool slowly before removing from the bed to prevent deformation.