I went back and re-read the thread. I still like the idea of incorporating a Dodge Magnum rear hatch. It's sort of perfect.
I think of the flat 12° topper as a compromise with buildability. The tough problem is the bed doesn't have taper in plan.
Maybe a flat-sided, simple-curved topper with a notch only wide enough to to accommodate the sight-line? See if you can move the mirror up into the windshield header and gain anything.
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