Convert chair from side mount controller to a hand held controller?

I can't imagine this would be difficult to do yourself. I'd buy a plastic box available online - search for a "plastic electronic project box" and choose a small black box to purchase.
Then remove the switches on the side of your chair and fit them in the lid of the project box making sure that you note where the wires are connected. Using some curly cable, extend the switch wiring from the side of your chair to the switches in the black box. Buy and install a pair of "cable glands with spiral tails" at each end of cable to stop it being pulled out of the box or chair.

Rupert
January 2020