My reading is that the 8x4x6 block has the long dimension across the vehicle.
8' long, 4' high and 6' wide would be most aerodynamic (crosswinds); but enclose that in The Template and your up around 40' long (I'm guessing). Look at the M-B Boxfish
If you don't like canvas pop tops, look for the old Econoline top
The sides fold down and the top panels relax into the that frame that extends to the front and rear of the pop top.