Simple letter/box opener design. The pictured one is the (printed) stl file. The revised file was not printed, but should work just as well. It is about half the thickness on the blade and half the thickness on the handle when compared to the original (printed) design.

Printed with generic purple PETG. Used variable infill to make the blade 100% and the handle 15%. Used variable layer height to make the first half of the knife at 0.07 layer height and the rest at 0.24 layer height. No support was necessary. Took about 8.5 hours.