The prototype is very different than what the product will be. The first try was just a coroplast sculpture that lasted quite a while but eventually was ripped off the car (while going faster than 80mph).
Here is a comparable product built with no consideration of improving FE:
The cargo box should open from the front and hinged on the rear so that when the box is open, the trunk can also be opened.
How close should the box be to the car? I think it should be about 4" away from the car, but there could be a foam gasket (pipe insulation) between the car and box.
The box should fit entirely behind the vehicle, there should be no part that extends past the vehicle to the sides.
Car taillights would be visible from the sides but probably not the back, so additional LED taillights are needed on the box.
The license plate needs to be moved from the car to the box when the box is in use.
I think a box of about 4 feet in length would be a good start. I would still want the car to fit in a normal parking space.
If you look at a lot of the small sedans, they have very similar designs and sizes. I think that a standard box could be sized so that it would provide good aero benefit to many different small sedans. It might not fit exactly, but just having a smooth solid body boattail behind the back of the car will improve FE, even if it did not perfectly match the profile of the vehicle.
I don't really have time to start a business, although it would probably be more financially rewarding than what I do now. I live pretty close to northern Indiana where they do a lot of this kind of fiberglass work so I could probably contract it out. But I don't really know how to start a business.
I do plan to build prototype 2 this summer out of plywood and epoxy. i have another job starting up in the fall and it is an hour commute so FE at commuting speeds is important. With the Versa, which is EPA 30mpg, I can hypermile into the mid 40's mpg (without any EOC). With the cargo box, I could probably get tanks in the low 50's with the same driving techniques, and could approach 60 with EOC. This is based on experience with the previous tail.
The reason that hypermiling works so well with a tail is that the tail makes it easy to coast long distances, and hypermiling is all about coasting as far as you can. Engine Off Coasting becomes a huge fuel saver when you have an aerodynamic body that can coast long distances.