A snap-on cover for a classic X-Acto series #1 knife (the common one). Prints nicely at .2mm layer height, and snaps on securely. Set shell and bottom/top thickness to 1.2mm. No infill should be necessary, but you may want to set it to 100% to be safe. A wide brim is recommended for good adhesion, it will peel off easily.
When putting on the cover, be careful and angle the blade perpendicular to the slots so it does not slip through the slots.
EDIT: v2 adds a twist to the slot to help prevent the blade slipping through the opening. I recommend using this one.
Public domain. Feel free to modify with or without attribution.
Designed in OnShape