Put your lamp on a thermostat. Zoo med sells a thermostat for around 20 bucks. It's called repti temp 500r, its great for single cages. If your having trouble reaching your desired ambiant temp them upgrade the bulb, as long as its on a thermostat its ok to have more wattage than what is needed (as long as you don't excede the limits for the thermostat (ex 500r is 500 watts)). Run this with a thermometer to get it perfectas the 500r does not have a lable to tell the exact temp. At the same time purchase at your hardware store a lamp dimmer. It's very inexpensive and will prevent you snake from getting burned by the UTH. Plug it into the UTH and put a thermometer directly on the substrate and get the set to the perfect temp. Aso this should fix the ambient temp in the cage on both ends (run the lamp on the hot side w/ the thermometer) and it should take care of the cold side. If not put SOME foil over the cold side to retain heat. This is the best way to solve your problem cheaply and effectivly.Happy herping!
(i know the thermostat sound complicated but it's not bad, just look it up and its a peice of cake...)