Shower knob busted, so naturally I made my own replacement.

This works just fine. I recommend a high infill as the heat in the valve as well as frequent torque on the inner slot may eventually strip out the inside and/or break. The first one had low infill and eventually broke (though after a few months) but 100% infill has been holding up pretty nicely.

May want to spray with a waterproof coating to keep mold from growing, but even without I haven't observed any issues.