You dont need a heat lamp at all, but if you cant get the UTH under the tank that could be a problem. I recommend you go by a plastic tub from walmart and use that for your snake, then go buy something else to put in that tank

http://www.youtube.com/watch?v=S-8YFl9ftks

Nicholas cage teaches you how to make a cheap and easy reptile tub, mine are kinda like this but modified slightly, I also use a homemade rheostat to regulate the UTH