If you stuck the UTH to the bottom of the glass it usually wont come off without ruining the UTH. the adhesive that they use is pretty permanant.
yeah you can build your own rheostat. All it is is a dimmer switch like one of those knobs that you turn to make the lights dim in your house, you can get em at walmart for less that 2 bucks. just hook that up to your UTH to make the temperature lower. (same concept as making a light bulb dimmer) I built one and it works, but the thing of it is I have to constantly adjust it to keep the same temp in the viv as the temp in my house vairies from day to night. a thermostat is definately better because it is automatic. (keeps the same temp in the viv no matter how hot or cold the outside temp is) I am working on building a thermostat but thats another project. If you are interested in building a rheostat for yourself I have the link for you.
http://www.cornsnakes.com/forums/showthread.php?t=19168