[freebeard]

Looking at #17, I wonder if the flexure is side-to-side or twisting.

The two possibilities I see are truncating the shell at the axle line and letting the wheel protrude, or adding skirts to the arm that would both stiffen it and clean things up around the derailleur.

I also wonder if adding a countermass ahead of the pivot point would act as a harmonic damper and affect the ride. ....but that's another matter.

edit:
It's hard to decide. If you take a tangent further forward, add a scoop at the top, pivot the entire tailbox, and use a 4-bar linkage to counteract the wheel, then you might get the best effect. It would need additional clearance for wheel hop, but weight at the back would be on a long lever arm.