Instead of supplemental heat, I just made my grow enclosure with side walls that hold in the heat from the lights. By making the front panel moveable and having a thermometer inside I can see how much of an open gap there needs to be to keep the temperature where I want it. Granted, with the lights on a timer so they shut off at night, there's an 8 hour period per day that they drop below optimal temperature but I presume that only adds a few days to time to sprout. I could just leave the lights on 24/7 till they were sprouting but I like to find ways to passively reduce the effort... set it up and forget it, or something.