A cold frame is not a small greenhouse. It is a lid over a bed. That sounds modest until you have lost a flat of peppers to an April night that was supposed to stay in the 40s.
On Sand Mountain, and down in the Boaz valley, the useful season for a frame is late winter through the last frost. You can harden off transplants for a week before they go in the ground. You can keep a short row of lettuce or spinach when a greenhouse would be more house than you need. You cannot grow a summer tomato in one. The lid is too low, and the sun in June is too much even with the lid propped.
The single pane frame is the one to start with if you have a single bed by the kitchen garden. The twin-bed frame is the same idea, twice as long, so trays and a salad row can share it. The raised frame puts the lid on a wooden box if kneeling is the reason you have been putting this off.
Set the frame where it gets winter sun and where you can reach the lid without standing in mud. A south-facing spot beside a fence is plenty. Weight the lid or use the prop when the wind is up. A frame that becomes a kite in a March gust is a poor investment.
If you already know you want to stand up and work, skip the frame and look at a 4x6 or 6x8 kit. If you are not sure, the frame is the cheaper way to learn whether you will actually walk out there in February.
See the cold frames. Questions go to contact@HillsideGreenhouse.shop or +1 256-593-2621.