Rather than buy a heat mat, I wanted to build something instead. My first plan was to build a wooden box and insulate it, but then I ran across this styrofoam cooler. I built a little shelf out of scrap plywood, with a 25w lightbulb mounted near the center. The only thing I didn’t have on hand was the cord plug. I’ll put my pots on the wood shelf, then cover the top with the lid, or maybe some plastic.
The 25W bulb is too much heat! I’ve had the lid sitting on top, slid to one side so that there is a 1″ gap of air, and the soil temperature is 105 degrees F. (outside air is 40 to 50 degrees F) I think I might have just cooked my seeds.
Installed a 11W bulb, placed lid on top, elevated about 1″ above rim of cooler. The soil temperature at the bottom of the pots was about 88 degrees F, and 78 degrees F near the top. I also ordered a line voltage thermostat (baseboard heater thermostat) to play with later.