If making a custom rack, maybe consider the Jim Merz style from the '70s:



Not the best picture, but if you look closely you can see the clamp on the blades started out as a close fitting hoop with two pinch binder braze-ons, which is then split. This is why Merz racks couldn't be made "on spec" and sold for people to fit to their own bikes; the fit of the clamps is unique to each bike. The hoop even has to be made to match the taper of the blade at that point, it's not a parallel-sided cylinder. It might take Merz-level fabrication skillz to make the clamps fit well enough, but this design is extremely well-proven, including around-the-world fully loaded camping with all the weight on the front rack.

Mark B