I mounted a 22" light bar to give me better light in reverse. It is sometimes flicked on for a half second 'as a warning' to aggressive drivers.
It is secured with two ubolts on the 1" square stock of my roof rack, wiring runs down the side of the rear window seal and then enters lower through the door seal with a drip loop before it goes in.
I have thought about designing movable lights also, but in the end, I'm fine with using a $15 high powered amazon flashlight for the rare times I need it. Personally, I decided to permanently mount my lightbar so I wouldn't get out to my jeep and find it missing one day.