Have you considered perspex /polycarbonate rear window..you could then drill holes in window covered with rain shields as per many race cars.
My experience of the S5 hardtop was that once we'd got it watertight, to prevent misting I then needed 4 or 5 vent holes in the trailing edge of the roof. What Kevfitz said :-)
Formerly Aero S5 #80 Currently 911 (992) Targa in python green
