I made a 2ft deep wooden "Box" inside my greenhouse. I filled in about 16" of fresh horsemanure, watered, allowed to sit for a week. Added 1 string of the old big christmas tree lights. I made a "Night" cover from 2" styrafoam insulation. This set up kept my chiles at 48F with an outside temperature of 18F. My previous set up was a pit frame, with styrafoam and wood walls. I almost lost my plants the second year, It rained for almost 2 weeks straight, watering plants was a chore in this set up. Good information on this topic in Nancy Bubels "The New Seed Starters Handbook" L.B.