I have tried 1 gallon milk cartons for winter sowing and didn't like them. I've tried bent window blinds under plastic , but that wasn't perfect. Now I am happy. I took a few of the "grow through" hoops that I use all summer long to stabilize peonies and large clumps of droopy flowers. I set them up in a row with some space between each one. The winter sown seeds (in 4" deep reuseable aluminum lasagna pans) were placed under the hoops and covered with a ventilated plastic cover discarded from a mattress store. They told me to take as much as I wanted. The edges are held down with rocks. It's a perfect little coldframe, very sturdy, and will not collapse. I can't believe how easy it was to set up. I now use cut up milk jugs as dividers between seed types in each pan.