Cool, but I'd be surprised if this actually works well - the shaft would surely catch on the shoulder bit, and the force pushing the shaft through is perfectly

Cool, but I'd be surprised if this actually works well - the shaft would surely catch on the shoulder bit, and the force pushing the shaft through is perfectly in line with the axis of the wheel, so there is very little moment turning the wheel. These can be fixed by: 1. Smoothing out the outside a lot. 2. Rotating the "guards" around the wheel by 45 degrees about the axis of the wheel, so the shaft enters at the top or the bottom of the wheel rather than in the middle. I can make a drawing if that isn't clear. Plus I haven't come across any low-friction 3D printed materials yet but there's not much you can do about that.