I use an electric space heater in either a closet or a wooden box I built in the greenhouse. I'm sure I get my seeds hotter than what is recommended, but it's fast and it works. In my experience, drying out is what kills plants or sprouts, which of course is accelerated by high heat, but as long as I keep things moist, everything's fine. I often starts seeds in vermiculite, which is very good at retaining water.
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