Found this on craigslist:
https://jacksonville.craigslist.org/tls/4791231640.html
57x38x20", 20 cubic feet of internal volume (34560 CI)
Taper isn't right, but I may find the right box.
Another:
67 x 35 x 16, 15 cubic feet.
EDIT: Does a premade cargo box exist that doesn't have a terrible aerodynamic shape? They're not even close to the profile.
Thinking building from scratch, or maybe using the wheel barrow front and building the rear from scratch would return the best results.