[RedDevil]

Now you've lowered the roof, you need new side and rear windows.
You've thought about Lexan (polycarbonate) I guess? Twice as light as glass and yet strong and safe (as opposed to other sorts of clear plastic - I know, I've been searching).
One possibility that might just work well with a severely hatched car like the Lupo is to make the rear windows curve in slightly towards the rear, allowing some air to flow between the rear pillars and the resulting (narrower) rear window. This should reduce the wake behind the car somewhat.
This is not feasible if you want to retain the possibility to carry passengers in the rear of course as it would compromise their safety. But I guess you've already removed the rear bench; then it should be fine.

On second thought; this shape looks like it could be extended with a nice boat tail perfectly.
Whatever way it goes, I like to see what happens (subscribed!)