The frame itself is just made from regular untreated 2 x 10s. A tip I got from market gardener and author Eliot Coleman is to tack a 2 x 2 strip around the whole bottom of the frame. This is what sits in contact with the soil and when it rots away you can just replace that strip, without messing up the whole frame (you don't really want the nasty chemicals that they use in pressure treated lumber in contact with your garden soil, so use untreated wood for a cold frame.)
Once you have a nice stand of salad greens that no longer need protection from the cold, you can move the cold frame to another spot on the garden and use it to start your warm weather plants like tomatos, peppers, etc. Yes, I'm running a little late this year, but I think I still have time for these late starts to bear in late summer and I'm just too cheap to buy plants from a greenhouse. Obviously I need to leave the cold frame open during the sunny days now, or the poor plants would cook. But I close it up on very cloudy days and at night and its 7 - 10 degree overnight temperature boost is welcome in such a cool year.Later this summer I will start a batch of salad greens in one cold frame and a batch of carrots in another to extend our fresh eating well into the early winter.