I found these cold frames on Craigslist for $40-$50 a piece. They are constructed out of salvage windows and have worked wonderfully. Every year I start my seeds indoors under grow lights and as soon as the weather permits I transfer them out to the cold frames. Squash particularly do well in the cold frames and when I move them out they are already putting on their first fruits. I do prop the cold frame lids during the day to allow pollination,
The cold frames have also been wonderful for early and late season greens. I simply keep a garden box of greens in the cold frame for year round salads!