This is just one small area where I use a sunbox to grow winter salad greens. It’s less than 2 feet deep and just short of four feet wide. This uses very inexpensive wood(2X4’s laying around the house), and a window frame that cost me $5 at Restore for the top. I’ve got two different kinds of lettuces, arugula, and spinach. I’ll wait until it gets a little deeper into the fall season before adding another layer of wood. If I put it on now it casts too big of a shadow at the front edge of the box. Maybe in 3 weeks it’ll be a better time. I usually end up leaving a small opening in the box by simply moving the glass cover back about 2 inches. I don’t every really worry about closing it all winter unless we have single digit temperatures at night, which we have on occasion.