Yeah that should work but its not as precise as a thermostat. It will still keep it from getting too hot.
Not sure about clearance. Not much since they give you those four little rubber things that are like 1/8" thick to hold up the tank, and they think that is sufficient.. as long as there is air flow under it i think its fine. It does say not to put it on wood surfaces and stuff but... I dont think it matters. Mines on wood. I think it can do something to the color but im not sure that it will, since its not getting extremely hot.