The hex-key/scaffolding joints are cast and therefore brittle. They might be so massive that this won't matter, but hit one hard enough and it will shatter, usually where tapped for the grub screw. The field repair would be epoxy.

Copper would bend if you dropped the bike but could of course be bent back to shape. Epoxy filling would stop it crushing. Field repair could be solder or epoxy or a new fitting.

Wood is a great idea but takes skill. I designed a sidecar body in wood, basically copying aircraft techniques deHavilland used up into the 1950's. Light, strong and simple to repair with more wood or fibreglass. The trouble is that to do it properly you need to make forming jigs to hold it and the right temperatures to get the glue to cure as you form the laminates. You could also get in trouble in warm climates with the glue failing as the layers expand. The results could be first class, but bolted channel only requires hand tools and welded tube at least leaves me with something I might use elsewhere or can e-bay. The sidecar body ended up as bolted, rivetted and bonded aluminium sheet and angle (more Vickers than DH) and worked well, but a box not a rack.

Plastics wise you want something more like a Nylon or PVC. Lexan type stuff shatters and cracks when placed under enough load or the wrong temperatures. Again, it's jigs to form it accurately, the one in the picture I would guess was heated and draped over a former. The bottom angle could do with a bigger radius maybe? If drape forming, a spoon shape like the old plastic canteen chairs is stronger that the shape in the picture which is more like what you would do with laminated wood.

If you want to mess about with tooling, you could also look at glass or even carbon fibre. Your rack could be bullet proof!

Andy