I assume that you are using your heat lamp in the middle of the enclosure to keep your ambient temp up? I would use a stronger bulb in your lamp and move it to one end or the other of your enclosure. Use a day glow to have more heat distribution or a spot for a warmer basking area and a little lower ambient temp. If you can put your lamp on a thermostat which will control the temp, then you will have an easier time when the weather changes outside to keep enclosure temps steady.