[Christ]

I wouldn't go with 15* unless you drive slower than 60-65 for the whole trip, you'll likely find that it's too steep.

If I get what you're saying, you basically want to build a Kamm back that is just an extension from the window/roof area, but ends past the deck lid?

As per usual, I always suggest about 12-13 degrees for a DIY project without access to a wind tunnel or a person just wanting to "test everything out" as it were.

Of course, that will put you out a bit further, and you're probably going to lose visibility through the rear window, something to consider... most clear materials distort at that angle, so everything will probably be a colored shadow that you can't really make out very well.

Don't forget to taper in the sides some, at least to match the tail of the car.