50mm x6mm exhaust clamp U bolt and some 4mm steel plate is doing the job just perfectly for me. I was able to adjust the angle to be perpendicular to the downtube with little aluminium wedges I made to go either side of the plate, and the shape to fit the oddly shaped tubing too.
It seems I only took a photo of the top part of the clamp which was ground to fit and then filled with epoxy to spread the weight.. But I did grind the U bolt too so it's flatter inside, and bent it to fit the shape nicely.