Hi stockrex,
15 degrees is the general rule of thumb on taper angle on a boattail-like extension. I have added a
SedanKamm to my car, it comes off a good radius at the top, and as it is now, the top half is 10 degrees and the second half is about 13 degrees, and there is good attached flow all the way to the end. It is 4 feet long and it gets me about 6% better FE at 60 mph and 12.5% less drag. I attached it with duct tape. If your car is nicer than my 1994 Suzuki Swift, you may want to consider attaching your boat tail to something more like a bicycle rack that attaches by clips to the edge of your trunk or hatch.
I have considered a "wing" like the ones in the pictures too. I just don't know at what angle the flow will re-attach if there is a sizable gap to see through, or how long I would have to make it so the flow would re-attach... the way I have it now works pretty well, but for a more permanent setup, I would look into something with more strength than duct tape (although I have had this on for a month now, and the duct tape seems to be holding up... original tape too). Look into
Basjoos' car (if you haven't already) and see the things he has done. Everyone serious about aeromodding eventually winds up going his route... smooth under belly, wheel skirts, boat tail... If I am getting some looks with the little bit of a tail I have, you can expect a whole lot more with a full out boat tail... like instead of having people look at your car, they might talk to you about it!