Just about there now. I found a couple of old “top hat” knobs in my parts bin. In the spirit of using stuff I already had, these seemed to fit the theme.
As I was wiring it up and testing it out one of the biggest problems I found was that the string anchor holes were too close to the bridge which gave a very steep break angle. This had the effect of canting the bridge saddle forward, so that the main weight of the strings was actually pushing against the back edge of the saddle slot. This meant that there was very little downward pressure onto the piezo pickup. For now I’ve implemented a bodger’s solution of a rounded bar which forces a much shallower break angle over the strings. I may just live with it and call it the Sustain-o-Bar™.