Do you have an intake fan for the box? If you don't, this is easily remedied by installing an intake fan as close to the bottom of the box and as far away from the exhaust fan as possible. Also, the intake needs to be drawing in at least the same amount of air as is being pulled out by the exhaust.

The last I remember, fluorescent lights didn't emit too much heat energy, but I'm not sure about CFLs. Aluminum foil also won't help your situation because it really isn't that efficient of a light reflector and reflects quite a bit of thermal energy. You could always paint the inside a flat white which should be a cheap and effective solution.

Finally, I'd say minimize the distance from your plant to the lights as much as possible. If what I remember about fluorescents and heat is correct, then your lights should basically be hugging your little plant. Remember that the intensity of light decreases with increasing distance due to the inverse square law (Intensity ~ 1/d^2 where d = unit of distance). CFLs don't have a high enough lumen/watt ratio to be effective any further than a few inches, so keep 'em close!

Just my 2 cents, but I hope it helps!