Now you can sharpen your knives on the bottom of any old kitchen mug. The unglazed rim is highly abrasive, and this device allows you to make use of it without cutting your fingers off.
I really liked the concept of benchapman's design, but figured I could make it more printer-friendly and adjustable. Here is an OpenSCAD version that is printed upright in order to avoid any need for support.
You can adjust the parameters in the SCAD file to match your mug and knife, but that should be unnecessary in most cases. I printed mine in PLA and found that the dimensions were too tight on both the mug and the knife. However, the joy of using thermoplastic is how easy it is to form and adjust after printing. I simply held it under hot water out of the tap and pressed it onto the mug, then used the knife to pry open the slots a bit. Once cool it retained this new shape, and now works perfectly.
See the pictures and benchapman's original videos if you're not sure how to use it. It's quite easy, and finally my knives are sharp!