These heater boxes/mini greenhouses are so simple and inexpensive to make. Just a 1"x4" frame with holes drilled in 2 of the sides topped by a piece of tin, with a switched cord with 3 outlets added. 2- plug in light bulb outlets are plugged into opposing sides and I leave the center outlet for plugging multiple heaters all in line. I use 60 (57) watt light bulbs, 1 on each side. As my seedlings sprout and get too tall for the tiny tray lids, I'll add a wire frame (I use those little wire fences. They have them at the $.99 store @ 3/$.99!) and cover in plastic to give the babies room to grow up!
Frugal Mini Greenhouses with Soil Heater