Standard Knife sheaths aren't big enough for the scimitar / butcher style knives.

There are small and large versions of the STL as I have two different sizes. The Small version has two styles - an open back and a closed back. The closed-back may need a bit of filing to get the knife to fit snugly.

The open-back style is a loose fit so it's easier to put the knife away - I find the tight knife sheaths can be a bit too tight to take the knife out, making them a bit more dangerous.

If you want it tighter, you can scale the print accordingly.

I print these with in the upright direction and add support on the build-plate only for the front-edge.

Print in .2mm to get smoother curves.