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Frank -- here is my understanding of box cavities, FWIW.
They come in two types, stepped and canted. The proposal combines the two. Compare the Vanagon example. The 10° angle supposes the air will follow the contour ala Coanda. The stepped box should have its free edge right where the canted version's is.
The open box works (maybe) because it reduces the cross-section of the wake, or because the air whips around and pushes on the back of the body. Witness the half-tonneau on a pickup. Why does that work?
The interesting experiment would be to test full- half- and no-tonneau cover versions. The half-tonneau is optimized by a wall at the front for a fourth variation.
In any case limiting it to above the lowered tailgate leaves 33% of potential gain on the table.
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