I would put temp gauge in there just to see how low it get in cold frame at night. I made one out for bales of straw and old window few years ago. Had it in the middle of the veggy garden worked fine. But set it up last week middle of April.

I use a cold frame every yr. I do have a heat cable in the bottom. Works great but it might be a little eary to start now. I put it sometime in April. you also have to make sure that it is covered w/plastic so on those heavy frost nights no cold air seeps in. I use bales of straw or hay. I have to check it several times a day as they really heat up when the sun is out. You might want to just use plants that you can live with out untill you get the feel of it. I love mine.
Good luck

Be careful! Even a little sun can cook plants,especially seedlings,in a closed coldframe. I don't usually put anything,like petunias, out until first week in April.Perennials would be OK,coldwise,but ventilation would be necessary.I used to put things out early,& invaribly it would turn cold at night.Then I would rig up a lightbulb & pray! Cover up with blankets,etc. Have had codframes full & covered with snow! Doris

I agree about the heat. It is worse that the cold. Fast and deadly. Those little guys can fry before you come home for lunch to water them.
At least with the cold, you can give some overnight protection.

Think how hot your car gets when it's sitting in the sun, even in the winter. That's how hot those little seedling will get, too.

One other thing to think about when you prop up the lid -- Is the wind going to be a problem? Use a propper-upper at each end so the top doesn't wobble when the wind blows. A notched stick of 1x2 will stay put when an unnotched stick will slip.

Thanks everyone. I'll wait till it warms a little more before I take out my seedlings. I look at the temperature in the coldframe in the morning and it's too cold for the seedlings yet. A lot of good advice you gave me.