Got your PM- sorry I can't remember where those old black steel buggers came from.
You could always build a pair, like these out of aluminum from a scrapyard. Just extend the bases to the width of your double rail setup.
Because the u-bolts will squash 1 " steel tubing (thin walls) and probably schedule 40 aluminum as well, I went to schedule 80 aluminum for the two siderails on the single rail catframe I built last spring. They don't need to be as tight on a double-rail frame, but keep an eye on it while you snug them down.