All very true Popshot. I just use what I have to hand and it works and It has served me well. I could use thinner metal it is true; but I am OK and happy in the knowledge that it won't bent under load or snap off at a critical moment, nor will my backplates go for a spin when I hit the anchors. While a hollow section and an inserted crush-tube would save some weight it isn't my top priority because I weigh about 4 times the weight of the trike.There are places where weight can be avoided and places where it can not. The clamping set-up you point out is a complex dual axis pivot relying on the friction of the clamps for fundamental integrity. As such the clamps need a large surface area and need to be substantial to generate enough friction. I could not find a lighter system I could make that would work without completely sealing the thrust bearing in so to make it unserviceable. If you could devise a lighter system please tell me. I do not purport to have any monopoly on the best ideas.
By contrast mounting a backplate is a much simpler task with no moving parts, where a 40x20 ish box section with a pair of reinforced holes would both be stronger and much lighter than bar. Bar has many great uses in bracketry etc but is rarely the best choice for larger pieces of chassis, though I very much understand we often use what we have rather than what is best. In my case I deliberately added weighty bits by reasoned choice here as I could not conceive of a better system. I have, on other occasions, used poorer solutions because I had them already.
If we were weight weenies none of us would use trikes but making something unnecessarily heavy has no benefit and ought to be avoided where you have the choice. Swapping bar for box is simple, lighter and stronger.