I don't know if you have the time or patience to build a proof box before you start, but Ed gives directions to build one in his literature.
DH bought a Coleman cooler, drilled two holes: one in the lid for a stem thermometer and one in the side for a low wattage lightbulb and put a rheostat in the cord so that I can control how brightly the light shines. It works like a charm.... I can control the temp perfectly between ~70o F and 110o, which is perfect for making homemade yogurt. It's a great solution.... and you could always use a styrofoam cooler if you don't want to put out the $$ for a more sturdy one. I use mine for bread, pizza, yogurt, sour cream, etc.... and it will hold the temp for days at a time, if need be.
Just a thought...