I found that this inexpensive and flexible cold frame has done incredibly well for me.
I live is Zone 9, light frosts, but not too cold. I've found this little protection and increased humidity does wonders for the plants.
I dont' usually have a lot of time for my plants so opening and closing it with temperature variations was a problem. My solution was to use seed covering cloth. You can buy it at just about any nursery. It's actually a plastic with thousands of little holes in it. The normal use for it is to put it over seeds to protect them from frost and animals and insects. It is so light, the seedlings will lift it up.
For the cold frame it means that air circulates and it doesn't overheat. In my case it is protected from the hottest part of the day, so I don't know how it would work in a full sun situation.
I used 3/4 PVC pipe for the frame. Pretty simple really. The most interesting part is how to secure the fabric to the pvc. I take 1 inch pvc, and cut it in half, lengthwise. This makes a clip. It has rough edges, so you can use a file to smooth it, or masking tape. Both methods work. You have to fool with it a little to get it right, but it works almost as well as expensive clips designed for that purpose.
