So we stop going off-topic in the Sedan-Kamm thread, I figured a new one should be made.
I went ahead and fitted my car with a 15" trunklid extension, modeled after the Fusion.
The trunk and spoiler have a 2-3* downward angle, and the sides of the spoiler continue the 3-4* boattailing of the trunk. The vertical end plates meet the corners of the car at about a 10* angle. The roofline meets the end of the spoiler at about a 15-16* angle.
We'll see if this shows any noticeable effect.