Rather than putting the grooves on a drum, which turned out to be pretty hard to engineer to any acceptable level of tolerance - this time I have used a flat perspex 'cartridge'.

By sliding the perspex into the slot, the arms follow the groove with a high degree of accuracy. I have added a little graphite (by drawing on it with a 4b pencil) to lubricate the motion of the arms inside the groove.

The new pencil holder is still constructed from 3mm steel wire, but constructed with a tiny bit more care. Also the pivots for the arm are holes drilled through 2 layers of rigid perspex which makes a big difference in terms of stability.

The only remaining issue which is noticable is that a slightly different duck is drawn on the forward and backward passes. However this is simply because the groove is about 1mm wider and the arm follower slides along different sides of the groove on the way up and back.