I would definitely add a fan so the temperature will be even inside the box. If you only end up running one bulb, then you will end up with hot spots. I used a baseboard heating unit with a safety temperature breaker, a fan, and an electronic thermostat. The thermostat will ensure that the inside temp is consistent regardless of the ambient temperature.
I did it like this: http://www.rcgroups.com/forums/showthread.php?t=1138304