I use an old toy box (20"x30"). I lay pieces of 2x4s (though you can use four bricks if you wish) and then lay a piece of plywoood on them, leaving a small gap around the edges. But before placing it in the box, I drilled a small hole in a wall and ran a waterbed heater cord through it, with the controller screwed on the side. Add the wood, plants and cover with glass. Once the seeds germinate, I replace the glass with a wooden top that has 4 23 watt CFL lights. That keeps the chamber warm but the heater can also be used if needed.
Mike