If you don't have AC and its getting really hot (above 85) it might be wise to just unplug the heat until it cools down. I'd say anything hotter than 95º and you might be facing some problems..
You shouldn't need too much in the way of humidity control in a plastic enclosure. They do a pretty good job of holding moisture. You most certainly need a thermostat if you are using a UTH.
Yeah a lightbulb wouldn't really work that well.