Just a proof of concept that I said I'd make available for others - no guarantee that the details below are correct or the design will work for you.
Here's the original and a guide to assembly: https://youtu.be/AzreNQiTvQE
You need an M8 bolt at least 40mm long, 4 M8 washers and an M8 nut. Two 608 "skateboard" bearings are used in the centre of the gear. You also need a pair of 13mm OD and 4mm ID bearings for the wave generator and a couple of M4x12mm screws - preferably pozi head to keep the head as low profile as possible - normal socket head M4 will interfere. Finally, you need an HTD 3mm pitch, 270mm long continuous (closed loop) timing belt.