That is indeed the profile of my car, Sven.
Thanks for your input! Sounds like a small Kammback might not be sufficient to make all that much difference. I did wonder if a 'shelf' jutting out from the top of the rear door (as you see on several cars) might give a sharper cut-off and reduce turbulence at the rear?
If I did build a boat-tail, I think it would have to be easily detachable so I can use it on the occasional long-run, but leave it off around town.
I lived in sub-Saharan Africa for many years, and a lot of station wagons were fitted with air scoops at the rear to direct air onto the rear screen in order to keep it clear of dust. Can't imagine it did anything to reduce drag though!!
I'm currently cutting some Correx to block the grills - but struggling to make clean and decent looking cuts!! Looks like a bit of experimentation and practice might be required...
I have a reasonably long run across France, Holland and Germany planned for the summer - be nice if I could get something working before then.