The two keys to this setup are
• 1" x 1" aluminum box tube, available from many hardware stores
• Two slightly modified Pedco Ultra Clamps, which I bought from a local camera store

I added two 1/4" x 20 threaded holes to the Pedco clamps, one on each of the two available surfaces. There's plenty of aluminum thickness to hold threads.

You'll notice a threaded hole in the box tube itself. The tube is so thin that this is only useful for very light-weight attachments. You could epoxy a nut inside the tube, but that'd be a bit of hassle.

You could, of course, add more Pedco clamps, or use some similar clamp, as long as it can clamp securely to the aluminum box tube and doesn't distort the tube with its pressure foot.