[aerohead]

Unfortunately,there's little,to no data for direct comparisons.
Boat tails are closed.Box-cavities are open.And box-cavities are typically straight panels with no plan taper.
AT Dynamics' 'Trailer Tail' does have plan taper and is open in the back.They have demonstrated a 5.54% mpg gain at 65-mph with a semi-trailer rig.
Sepp,one of our members in Belgium reported a small mpg penalty for his Nissan pickup aeroshell,when the back was open.
Perhaps you could do a cardboard and duct tape rear bulkhead and quick tests to see.It would be important data.