Regarding good and not-so-good heat mats. I have several of the Hydro Farm small (single flat) heat mats. They seem to do a very good job of keeping things around 70 to 72 without benefit of a thermostat. Actually, using the small ones with a thermostat seemed to work less well than without. I also have two of the nice big 4-flat heat mats. They seem to work better with the thermostat. And don't believe what you read: trying to use the small heat mats and the big ones on the same thermostat just doesn't work. Seems like it should, but it sure didn't work for me!
And then there were the heat mats I got for cheap from Harbor Freight. If you want to start corn indoors, they work great. Corn LIKES to be hot. Otherwise, I had to prop the flats up nearly an inch above them. But they were only about $12 each. Another downside of them was that the covering on the mats appears to be somewhat porous, so I'm very careful with them.
Now these are being used in a normally unheated greenhouse (unless it gets really cold, then I turn on an oil-filled heater to moderate the chill) and I put domes on the flats. And I set the mats on top of 1-inch thick styrofoam sheets to minimize heat loss from cold air underneath.
