MountainBikeRoamer Mike T. ,
It would have taken me at least 6 maybe even 7 minutes to doodle those doodles.
Very nice preliminary sketches! can't wait to see finished drawings.
That is the very way I envisioned the swing arm mounts in the storage box.
in addition if the swing arm were to not only mount to the top / surface of the box but also be located / fastened to the bottom interior of the box IMO would prove to be a very strong pivot point. (spindle same height as box)
I like the looks of the "Thin bumper cap" looks like the type of bumper that would be on a Sportsmobile.
Lastly an idea for a single swing arm / centered spare tire mounted lower to retain unobstructed view thru rear windows. used with your bumper cap with the wide recessed center section.