An AudioVox CCS-100 Electronic Cruise Control
This gizmo cost less than 100 USD, but has been priceless for my wife. She has an injured right wrist and was about to give up riding. Throttle locks helped a bit, but off the interstate the thumblock just added more drag and stress for her bad wrist to overcome.
The electronic cruise holds a perfect steady speed, turns off if you touch either brake or the clutch, and has enabled her to keep on riding. Priceless!