And to help hold heat in the tank, I’d look at insulating it. The easiest would probably be foam-core* (poster board with a foam center) that you can pick up at Wal-Mart or any craft store. I’d do this for three sides of the tank, plus one piece covered in aluminum foil for the lid...just leave enough room for your heat lamp and a bit of ventilation. This should help boost ambient temperature and the covered lid will help hold in humidity.
*You can also use cardboard, bubble wrap, or rigid insulation (found at a hardware store, looks like stiff pink styrofoam).