This was made using the revised AST (aero streamlining template).
I started by taping a strip of aluminum (carpet edging, I think) on the roof and pulling it down to get a curve.
I'm not expecting big things here, because the Civic's stock rear profile isn't too bad.
Wiki says the '06 Civic sedan has a
0.31 drag coefficient. (No source given.)
I haven't yet tuft tested the car's rear glass, but I wouldn't be surprised if there
isn't recirculating flow at the base of the window as I've seen on some other sedans.