My wife loves to buy these 3 pack multi color knifes, either Wiltshire or cheapy brand from Reject shop becuase they have sheaths.

My children love to loose the sheaths.

This is a remix of one found on thingyverse, using a ruler, i simply measured the approx length/width/depth of the knife and adjust in the slicer scaling by turning off Uniform scale, few trial and errors and this is probably the best fit i had, its not tight to get in, but stays on without being too loose.

This may vary by knife, but as above simply download and adjust scale to what you need.

(example of knife this should fit straight out here, → Wiltshire Multi Purpose Knife with Sheath, Multicolor (3 Pieces Set) : Amazon.com.au: Home Improvement)