I made one this year and it works. It doesn't seem all that warm, but a little heat does a lot. A 21' string of 100 Christmas lights, section of plywood the width of the seed trays, and 3/4x3/4 to 1x1 strips to build the channels where the lights are fastened is all that is needed.I dammed up the space around the lower edged of the seed trays with rags to hold in the heat. Plants began popping up within a week and we had several very cool nights. There is no heat on my porch unless I open the interior door and the woodstove heats it.