You might check out
AeroPACK Cargo Carriers
FWIW, it doesnt appear that it would work on a Prius due the suspension
straps pressing on the somewhat fragile
rear spoiler.
Here's a
German version that appears to be better engineered.
In either case, I doubt that it would enhance aero.
At best, when rigged on the back of the vehicle, they wouldn't degrade
areo very much.