This happened a while ago, but I'm just now getting around to posting the pictures. We wanted to insulate one of the raised beds from the cold so we could keep seedlings in there to overwinter and get an early start in spring. We'd liked to have harvested cool weather greens all winter, but didn't get the plants developed early enough, so that isn't a possibility.
We made the cold frame so that we could easily install it each winter and break it down each Spring. We drilled brackets into the inside of the raised beds.
Then we used PVC pipe to make the skeleton of the cold frame.
Then we draped polyethylene plastic sheeting over the whole thing. We weighed the sides down with bricks to keep it in place.
The right side was mcGyvered since we didn't have enough plastic sheeting to have it be one continuous sheet (we'd used too much for drop clothes on house projects.)
Inside there are spinach, kale, and arugula seedlings. There's also a sad little enormous broccoli that may or may not ever produce. The sheeting will protect against frost, but let enough light in that we should be in business pretty quickly for next season.
The seed catalogs came on November 28th, so planning is already underway.