[aerohead]

I guestimated:
Height ( H ) = 103.25"
Ground Clearance ( GC ) = 10.5" at the front
'Box' height ( h ) = 87.5"
We know the width ( W ) = 84.0"
Tires , around 285mm ?
Box upper side radius, about 12"
Tire wake, about 1.636 sq-ft
Box wake, about 50.6118 sq-ft
Cd, about 0.34
CdA, about 17.764 sq-ft
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PhD Jeff Howell et al., at Loughborough University, suggest that lowering drag by some percentage, is achieved simply by lowering the wake size of the 'body' by the same percentage ( not the total wake including the tires, just the body wake ).
* If the Zevo's body presently has a 50.6118 sq-ft wake, then, theoretically, you'd hit your 20% drag reduction goal when the wake was reduced to 40.489 sq-ft ( call it 40.5 ).
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In 1995, Texas Tech University's engineering magazine published an article about their 'Tailwind Project,' honcho'd by Professor Carver.
The cover photo included their 1/12-scale, 18-wheeler in the wind tunnel, with the General Motors' 'GM OPTIMUM Tail .'
This tail was designed for the upper edge radii, as on Zevo, and embodied a 20-degree rear top downslope, 10-degree side body in-slope, and a 10-degree up-sloped diffuser.
For 'non-curved' boat-tail technology, it's likely that GM's tail represents the 'best' design extant.
All of it's edges are 'softened', including the vertical trailing edges. It's worth seeing, if you don't have it in your library.