I realize this is not a good picture, but it's the best I could come up with. Sears sold these back in the 70s and 80s. It's threaded rod with metal corner pieces. Wing nuts on the threaded rods tighten it up. They claim it could clamp from 4" up to 36". I grabbed one for $2 at an estate sale a couple of years ago. I haven't used it yet, so I can't say how it works. I think an advantage might be that it can clamp something a little out of square because of it's adjustability. I bet you could make something like this to suit your needs with wood blocks for the corners and whatever rod length you want. Threaded rod is pretty cheap at a hardware store or big box.