I am having really good luck with the first product on this page: (Mine came from Ebay for a little less.)
http://www.reptilehavenonline.com/cgi-bin/supplies.cgi?p=thermostats
The mat is thin, like a piece of paper and the thermostat works well. I put the mat dircetly on the bottom of the cage and placed a very thin piece of plexiglass over it and duct taped the glass to the bottom. It is a big cage, and I also have a heated cavern and 2 heat rocks that stay on 27/7. The thermostat keeps it at least 70 at night, no matter how cold I let it get in the house. During the day the basking and florescent lights keep the hot side in the 90's.
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Valerie
We may have all come on different ships, but we're in the same boat now.
--Martin Luther King, Jr.--
God bless the USA