I have more of a 3 smartie car but have been thinking of doing the same thing, building a Kammback to connect the top of the rear window to the trailing edge of the trunk. I sketched up a picture and and I get an angle of 16 degrees which from what I read is too steep to keep the flow attached.
I guess the next option is to try a kammback similar to the ones shown on the civic and metro hatch back. I can't imagine it looking very good but I'll try to sketch something up.
Ian
ps, this isn't my Neon, just one I found on the net. Mine is so much nicer with matching wheel covers....
