You're using a tank right? Like a fish aquarium? Then you need to almost completely cover the top. Cover more than 3/4 of the screen with foil duct taped around the edge. Leave a gap on one end or a small gap on either end for air circulation.

This will help hold in heat and humidity. Tanks are bad for snakes because they are not an enclosed reptile/snake cage like a Boaphile etc.. If using a tank you should make it as enclosed as possible.

If you do this and the cool side still won't get close enough to 80 them your house is too cool and you will need a low wattage ceramic heat emitter to compensate. I bought one off eBay a few years ago for a lizard tank. They last a couple years easy if they are not jarred around and they put off a lot of heat. Just use a good lamp with ceramic fixture to screw it into and the tank would be plenty warm. You would even be able to lower the temp on your UTH a little.