Since you mention not wanting to spend too much money; mist the enclosure manually for the time being. The wattage required to heat enclosure will be directly proportional to the room temperature and the enclosure size. If room is not too cold; you might be able to get away with only a ceramic heat emitter.

My recommendation is to get a dome fixture with a built in dimmer (Fluker's or similar) and get a 60 watt emitter, like Colleen/Jerrod stated. Then cover your whole screen top with aluminum foil securing it to top with electrical tape or similar. Place your fixture on top and using a marker, draw a line around it's perimeter on foil. A scissor will work well to cut the marked out hole; just make it larger by around an inch. Once connected you can use the built in dimmer to adjust the heat output.

Once adjusted; the above idea will work fine as long as the room temp is stable. If not; then you will need to invest more money for a thermostat type automatic controller (i.e. Hygrotherm). If you can dial in 82-84F daytime with a few degree drop at night that will be fine, good luck !