First we built a false-bottom, 7 inches from the top. We left 4 inches for soil and 3 inches for growing room. The soil will settle a bit, but we didn't want to put the plants too far below the lip otherwise they would get shaded out too much in the morning and afternoon. This one is intended for mesculn and spinach, so they don't have to get very tall before they are harvested.
We lined the false bottom with the ever useful feed-bags. It is semi-permeable, and will allow excess moisture to drain away. We are hoping that the insulated air cavity below the plants will act as a thermal sink, heating up in the daytime, and holding some heat over through the nights.
We put a layer of fresh horse-manure down first, like a hot-bed, to provide a bit of heat and some extra fertility when the roots reach that depth.
Then dressed the top with a layer of well composted goat and sheep manure and potting soil.
This size freezer allows space for three rows of greens, 9 row feet in total. We planted two rows of zesty mesculn mix and one row of mizuna mustard greens. We have too many flea beetles in the garden come spring to plant these kinds of mustards, their favorites it seems. Even with row covers, the flea beetles come up out of the soil underneath, and really give our brassicas a hard time. But there's no flea beetles out now! I am even beginning to think that I could use the freezers as brassica sanctuaries in May. We usually transplant our brassicas to the garden in mid-May, but the tender transplants are quite vulnerable to flea beetle damage. If the flea beetles come out of the ground and have nothing much to eat, then they won't breed as rapidly. So I may try transplanting the young brassicas into the freezers mid-May, and let them grow a bit stronger, and past the prime flea beetle season, and put them into the ground as bigger plants in early to mid June. Too much transplanting can put a lot of stress on the plants, but if it is less stress than the pest damage, it may work out in our favor.
Finally a few sheets of glass on top to complete the cold-frames. Under a clear sky, it easily got to 35C (100F), and the temp can be regulated by sliding the glass sheets to leave vents. The day after planting, we got a snowstorm (another foot of snow!), and simply put the freezer lids over the top. It was holding at 3C (35F) with the lid on.