I don't really know. I've been offering counter-examples for consideration. It would take hours in a wind tunnel to separate fact from fantasy.
Given your departure angle requirement, retractable skegs sounds reasonable to me. They could have a polyurethane skateboard wheel at the tip. Sort of wide-spaced wheelie bars with fill panels. They could be shorter than the tail overall, or canted like on a 1961 Oldsmobile.
An important consideration would be a smooth transition from the Prius proper onto the added structure of the boat tail. I haven't done a complete boat tail myself, but the 1:1 scale study I did was to find the point of transition. It turns out on the Beetle with a Tropfenwagen tail, it's a V- or U-shape across the roof instead of a straight line. Instead of cardboard, I used 1/4" Redwood benderboard ripped in half. This could be extended to a wooden-canoe-like construction.
edit:
Looking for something else I found this old thread,
A paper on aft-end underbody diffusers, and a Summary at
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