What about a small camping lantern, with a rechargeable battery? I don't know about guineas, but with my chickens, I treat train them - throw some scratch in the coop just before bedtime, keep them occupied for a bit, then they're already in.So my Guinea hens will not go into their new pen at night because it's too dark without a night light. Will they get used to going in on time and not needing a nightlight?
I sadly have no electrical outlets nearby.