[aerohead]

The PRIUS-V already suffers an aerodynamic penalty, due to the lack of meaningful slope in it's roofline, compared to the PRIUS.
Cd 0.29 vs Cd 0.25, a 16% loss.
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If you yanked the rear hatch, and constructed a hollow tail section, extending the existing roofline down, plus adding to the plan-view body camber and baby boat-tailing, plus some 'slow' diffuser down below, you'd really have something, depending on it's final length.
And if you 'light-weighted' the construction, the added mass might not equal what was removed by deleting the OEM hatch.
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A remarkable amount of drag reduction occurs within these curvature 'origin' areas of streamlining.
I suspect that you'd easily 'beat' the regular PRIUS' lower Cd.
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I looked for 'Champrius' here at EcoModder but couldn't easily locate it. It's a 2009 PRIUS with extended tail 'camper' space inside, and pretty terrific mpg.
It's easy to find at GOOGLE.
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You can probably find a 'blueprint' for the V online. It would show, in 'true-length' all the actual geometry of the car, and an enlargement done at a local copy center would allow you to 'explore' the 'construction' on paper, to see what you think you could stomach.
LIGHTYEAR Zero is currently leading the pack on low drag. A careful study of it's aft-body, in different views would be invaluable to you.
The relationship between fastbacks, squarebacks, 'geometry', and drag are well understood ( notchbacks: forget it! ).
Your Prius is a squareback and will respond well to squareback 'known quantities.'
Contrary to some prevailing views, you'll essentially be at a 100% confidence level of attaining attached flow and drag reduction at any chosen body length.