Thanks Shapp! I am a machinist, and I have all of the NC programs and tooling to produce more of the fittings if someone were interested.
The frame can also be easily converted to a day frame, using the cross bars for the double rails. I am in the middle of finishing up this setup at this time, when I am (next couple days, hopefully), I will start a thread and post some pics of the fittings, the frame setup in both configurations and give a good description.
Penguin, I just noticed that you were looking for a longer box, and I think Frontier also makes a 24" x 12 box that might work for you. A larger bevel would be required, but would probably work just the same. On a cut that size, curving the cut to follow the tube would minimize wasted space, just bend the sheet to the curve when welded on. Would look pretty sweet.