I have used all sorts of creative shit for bottom heat for seedlings!!!
My current spot o' choice is on top of a properly-functioning ballast- one that is not overheating, lol) but I've also put them in trays over the pilot ligth of my gas stove, and back when I was a cellardweller, on top of the hot water heater. hahaha the only worry is light, so if your heat is good and your light is good, COOL.