I think you'll find that most air will still flow over the car. Take this spoiler for example, it isn't flush, but it redirects air such that drag is supposedly reduced. If the back of the boattail is sealed, then very little air should get in, and even if it isn't I doubt there would be very much lift. For example, airfoils result in lift because the area on the top is greater than the area on the bottom with the same approximate air speed, but in this case, there won't be very much air flowing through the opening, and the area on the top and bottom will be the same. YMMV.
Originally Posted by FormulaTwo
I think if i could get that type of FE i would have no problem driving a dildo shaped car.
Wait 'til you drive through rain and the whole thing get filthy. How will you reach in there to clean the windows?
I was thinking you'd want to hinge it so that you could a) access the window for cleaning, and 2) access the trunk. The hinge could be at the structure's leading edge and latch near the rear spoiler/trunklid. This way if you unlatch it, you could lift it up to open the trunk. Also, if the hing was at the rear, and the latch was at the front, and the latch failed for some reason, the think could get ripped off by the slipstream.
Reality is merely an illusion, albeit a very persistent one. - Albert Einstein