I have photos of today's racks. Maybe later.
The ply bracing is directed at the 'cheap' rack proposal. Cheep is the $11 Special of 10 years ago but here 'cheep' is naborhood racking similar to the commonly available Blackburn line. These racks wag and rub or are there suggestions for nayborhood racks NOT wagging and rubbing.
A collateral question is what specific design features prevent wagging and rubbing ? Rack tubing strength or strut triangulation...and if triangulation exactly what geometry are you writing of ?
I should look at BG's racks again...BOLT ON and strong tubing would eliminate wag an rub fersure yet retain simplicity which the top Tubus does not.
The ply method is more monocoque. Nothing wrong with a thin aircraft ply panel....out of the wind. But the bags are on it so what wind ?
The main rack here is old recycled firstcut ply, '58 countertop, on aluminum...I forget which rack but cheep with U-bolt connectors to the seat tube.
Rack's dropout bottom bolts are grade 6 6mm. I first thought a hardened bushing was necessary but no wear there after 10 years. The bolts need replacement t maybe 5000 miles, after 5 the bolts will break on down the road somewhere. Bolts are redlocked so no jiggle wear to the frame. The red comes apart by wrench after lube and vibration.
VT, would you expand on your thoughts using ply panels for the CDT ? I am onto the NM section. Not as a gungho thru or be damned rider but going over the terrain to see what. Looping is acceptable. I can 'turn back'...looks different that way