So, as I lay in bed this morning I started thinking about this again. It strikes me that I could get away with not having the central hole cut to mate to the shaft directly - i.e. curved and flat in different sections. A hole that would match the overall circumfrance of the shaft could then be secured by two grub screws, one onto a flat face and the other anywhere apart from directly opposite it. Sadowsky knobs are attached by two grub screws and are remarkably stable. I have thought of 3 screws at different points from the side, but suspect that 2 would do. Obviously there would be the screw through the end as well. Even though that might not actually be necessary. I don't know and am open to suggestion here. My reasoning is that fabrication would be a whole lot easier. It would not be as elegant a build, but I think the trade off might be worth it. Of course, the CNC programme might be sitting there thinking "why bother not doing it right with one screw, it is easy for me". I would appreciate your thoughts.