Just get a regular clamp on light and put a 25W or equivalent bulb in it. Even 25W is plenty enough to be able to see a fretboard. Clamp it to something convenient and up high. Use AC. Carry a long extension cord and 3-in-1 divider. Personally I use music stand lights (the real kind, AC powered, not the battery ones that run out midway thru the third set) because I've got five or six of them, but you can probably find all kinds of useful lights at a well stocked hardware or home furnishings store.
Ha ^ this post just reminded me of a story that's kinda funny in hindsight...
Most of the places we play here in Seattle are actual music venues constructed with the intent of having live bands, so we've never had to deal with jenky electrical wiring or dark stages unless a performer requests it.
Our first trip out of town however, was at a place that was just starting to have live music, and I guess we came early in their 'learn as you go' journey. It was an odd lineup of us playing post punk in between a gothic sounding electroclash duo and a metal band.
After realizing that the large stage was darker during the first act than the dimly lit dancefloor, I approached the guy working sound and asked if he had ANY kind of lighting he could hook us up with, to which he replied "Sure."
When it was our turn to go up there and set up, he disappears in the back and returns with two worker clamp lights with one 25w red bulb each and places one on each floor corner of the 35ish' stage. Now my bass has a bare fretboard with only the tiny side dots on the top side of the neck, so when I'm looking at it all I can see is a vague outline of the neck barely bathed from the bottom in red which really didn't help all that much
Luckily I often attempt to practice with my eyes closed, and at some point early on I did manage to locate a sweet spot where a reflection of something else was casting a very tiny beam of red light to the top of the neck if I stayed there and didn't move around too much. If I recall we ended up playing a pretty good set that night.
Since then I've been considering ordering something like those glow in the dark stars that were kinda popular to put on your bedroom ceiling a few decades ago, to put atop the side dots or on the fingerboard and then just peel and Goo Gone them off after a show, but since we've not played many unknown venues since then I never actually followed thru with it.