Another option-
I was at Walmart yesterday and they were selling magnetic-attach worklights in the automotive section for $14.88 each. They were rectangular, had a pretty long straight cord, and plugged into a lighter socket. Round magnets were strong enough to hold vertically.
Mark
Oh, and they also sell weather-resistant power sockets. The wires are "molded-in" to the assembly and there is a gasketed, spring-loaded door over the hole.