Super Droid Robots replied to my email. They said to just order another motor wiring kit and ask for just four feet of cable instead of all the other stuff. Placed the order tonight. It is going to cost about the same to ship the cable as the cable itself.
Stopped by Home Depot and picked up a nice black shelf to mount everything on for $9. It is 4 ft x 1 ft and just over 1/2” thick. The cart should be able to travel the entire length of the board, minus a small amount on each end for the shaft supports. The cart is about 7 inches deep, so that will leave about 5 inches at the back of the shelf for the power supply and sbRIO. The sbRIO will be a tight fit, but it might be able to squeeze under the slide unit.
Picked up some shim foil that is 1/1000” thick to wrap around the motor shaft and tighten up the gear. Cut a narrow strip about 1/2” wide and wrapped three times around the shaft. Gear now fits perfectly with no wobble.
Tested the encoders today. The two I thought were good worked perfectly with no problems. The one with the shaft that turns roughly doesn’t have any signal, which doesn’t surprise me. I guess the guy just threw it in with the others to get rid of it.