Only tested with PET-G, PLA might not be flexible enough.

Hole for a 6mm Allen wrench.

Raised model has two small sections raised 0.6mm above the rest of the model. Print in different colour for best results.

This harmonic drive consists of 3 parts, the outer ring, middle flexible gear, and inner wave generator (that is what a video called it, I still think this is magic).

The outer ring has 38 teeth while the flexible gear has 36. The inner ellipse makes the gears mesh at two points. Turning the wave generator causes the points to move and the flexible gear to rotate by 2 teeth per revolution of the wave generator.