Ethernet cable arrived today. Cut off one end to see what kind of shielding it has. It looks like it has plenty. Each pair of wires is wrapped in foil, and the whole thing has a braided wire shield around it. Should work great as a replacement cable for the encoders.
Ordered several pieces of aluminum from Online Metals. Ordered one foot pieces of each of the extrusions used to build the cart just in case I need to rebuild it at some point. Ordered a four foot 1/2” square rod to attach the rack gear to the shelf. The rack will be bolted to the bar, then the bar will be screwed down to the shelf. Also ordered a 3 foot 1/2” diameter bar to use as the pendulum. Even with shipping, the prices were much lower than buying all the metal at the hardware store.