These are really fun! This design could possibly be improved but it does work pretty well.
The newest version files are here in AI, SVG and Corel formats. It’s a pretty simple design.
Here are the parts. Basically a larger square overhangs the smaller creating a channel with partial cover that the little “cars” can slide in, but not pop out. Also there’s one more little square piece that goes on top of the little cars like a gasket to give the handle some space off the top so there’s less friction and catching. That’s in the schematic, but not in the photo at the right.
Careful glueing and paraffin wax give good results, but there are multiple ways to get problems. This one is working smoothly and you can use the hole in the handle to draw an ellipse. This is an early video, and the ellipse below came out even better with a better hole for the pen.