If all you want is some form of throttle lock, just glue your bar end weight pieces together and fit a rubber washer between the handlebar and end weight. Dial in the friction you want. Works fine, been using it on my bike for ages.
If you want a true cruise control Audio Vox I believe makes one that can be installed.
There are also some hand stretches that can help with the numbness. I.E., hands palm to palm in front of your chest, raise your ellbows, etc.