Okay, I made a quickie paper model of the boattail I am thinking about for the xA (and I had a paper model of an xA!):
It adds almost exactly 1/3 more length to the car, and comes close to ideal for angles: 4degregrees underneath, 9 degrees on the sides of the lower part (to keep some width at the back) and 15 degrees on the top and sides of the upper part.