Yeah, that's what I was thinking of. The other idea was to continue the curved lines of the roof fairing into a rounded tailgate Kamm and/or spoiler-type tonneau top. I think a rounded arch-style tonneau top, sloping downwards, could look cool and un-spoiler-like enough not to be laughed at. I could make it look like some kind of fancy executive beer cooler.
Actually, since I couldn't let a day go by without trying something new, I did stick a tiny flat extension on the back using the only scrap of anything I had lying around. But it looks so stupid the way I did it that I'm embarrassed to post it, and besides, it made no difference in my commute. To mate the air off the cab at 12-13 degrees, it would need to be long... I think my trig told be that it'd need to be 12-24" past the tailgate to have a chance. I think my best choice is to raise it a little , and make up some of the angle with fancy curvature from the cab fairing.
One advantage to modding a truck is that, if it's designed cleverly enough, I think a tailgate Kamm could fool people into thinking it's some sort of towing apparatus.