Cold frame was built early last year. It has worked very well this winter, allowing tomato starts to get full sun during the chilly months here in SC. The main problem is it efficiency. On a sunny 40 degree morning the temperature will soar to over 110 degrees if the lid is not opened very wide. I don't think an automatic opener would help very much as that would only open the frame about ten inches. So each morning I wait until about ten a.m., place the tomatoes inside, and open the lid about 30 inches. Here is a photo.
My one largest tomato that survived transplant problems now has numerous blooms which are yet to open. Some years I've only planted seeds by this date. Hopefully we will set an new early tomato record this year. The largest pant was up potted to a three gallon nursery pot, well ahead of any previous year.