Not perfect, but better
 

Boat Tail with movable flap:

A better description:

I don't know if you caught it, but the most recent post from kach22i in the Random Wind Tunnel and Smoke Pictures Thread Tells the 'tail' of the Porsche 917.

http://2.bp.blogspot.com/-1mAMhLeu2O...7-page-6_2.jpg

The proportions and goals are different, but maybe there's something to be learned. What your hinged plate reminds me of is the Dauntless dive bomber dive brake.

http://www.davidpride.com/Aviation/M...APS_03_090.jpg
http://www.davidpride.com/Aviation/MAPS/images/MAPS_03_090.jpg

What is the use case? Is this a high-speed vehicle that deploys the flaps for landing?

Yeah, this is my old 15 degree boat tail, with a lower flap to give the 15 degree bottom line in a way that can still clear (with a scuff pad) ramps. Like I said, it's not perfect, but it's an improvement over the stock rear of the Prius,(AND it can hold some cargo, too!)