I saw someplace where a gal was going to use the waterbed heaters instead of heat coils to start her seeds.I have been getting some on freecycle but have had to take mattress also.I am thinking about using one or two of the mattress filling half full water and sitting my trays on them.I lost my greenhouse last winter and havent been able to get another one.I am wondering if I take one of the frames and sit on the ground under a old swing set and cover the frame with plastic would that work?Thanks