Actually, you've got me thinking more and more about your/our approach. We'd
need some way to "keep" the cables so that they'd be sure to engage the
groove on your ellipse or any of my oddball shapes. It could be so simple to
make the diamond shape by using a couple of plates held apart by spacers,
with the cables running between the plates and attached to one of the
spacers at the fore end, the narrow axis of the diamond established by a
couple more spaceers, and the other end of the diamond by another spacer.
Two plates, four sets of bolts/spacers, and some cotter pins outside the
cable runs to act as cable keepers. The bottom plate could double as the JB
belcrank....