RHP is about the only other option, since it's a wooden enclosure and the wood is 3/4" thick, a UTH isn't practical.

I was thinking about this dog house heater. I emailed the people who make it and he said they sell a lot of them to reptile keepers. It's got a thermostat on it and isn't real expensive:

http://www.houndheater.com/products.php#houndheater