I'll reply to two posts here.
The rheostat idea will work fine but like you said it will get hot and should not be in the area behind the refridge. The autotransformer is a good solution but expensive. My sugestion on using the Radio Shack transformer is a cheap method of building an autotransformer.
The 600 watt dimmer should work great as long it is wired only to the heating element as you have done. The only problems maybe noise in AM radios and the voltage you measure may not be correct. If you have a standard digital voltmeter the reading will not be correct (it maybe close, the higher the dimmer setting the more accurate the reading will be). If you have a true RMS voltmeter the reading should be correct.
I may try the 600 watt dimmer methode.
