Very interesting.
Observations of an aerodynamics neophyte:
The 4-seat shape had a max diameter of 5 ft. and a useable length of
12-1/4 ft., a fineness ratio of 2.72.
In the Conclusions, the authors suggest that max efficieny would occur at a
fineness ratio of 6, for a useful length of 30 ft. with the same 5 ft. diameter.
I can't help but notice that the cross section of the fuselage shape, Fig. 14,
looks roughly like two flat-bottomed air foils placed together with the trailing
edges facing forward. In essence "flying" backwards. I suspect this
observation has little or no practical meaning.
I also note that the section of rotation is somewhat replicted by the roofline
of the Gen III Prius from the wheel wells up:
I guess that isn't surprising given the Prius' low Cd.