Hello Kent,
I did not know the exact use. Please accept my appologies for jumping the gun, I had a horrible vision of people assembling an entire roll -cage and clmping it together with loose clamps
I would have suggested a slip fit onto a stub tube with a pin similar to what it sounds like you have done that would put the pin in double shear which a suitable size pin would handle with ease especialy if there was a tube welded in the stub for the pin. Replace the pin with a bolt and a nut welded externaly would work to.with a lock nut and wire as well of course. Givin the fact the rear bar needs to be removable and where the clamp is your solution is fine, the only other approuch would be square plates bolted together and placed between the tubes and each welded to the adjacent tube in line with the tube axis.
Different strokes for different folks.
I love your clamps by the way and I will be copying them for all sorts of things.
Cheers,
-Barry.