jamesqf -
Here's my idea. Crack the rear passenger window down maybe 2 inches. Get something like a half-circle pipe. Aim one end just barely into the airflow. Aim the other end at the back of the driver's head.
My idea is that you will lose some aero, but you may get the best "bang for the buck" because you should be getting pretty fast air flowing onto the back of your head (as opposed to your face, which would make your eyes water).
You could test this by having someone hold the "half-pipe" to the window while aiming it at the back of your head.
PS - Maybe the pipe should be conical, large intake to small outlet.
PPS - It
might be more efficient to have the pipe cut in half like a Jai-Alai glove so that you capture more incoming air through the window, as opposed to the diameter of the pipe :
The Cesta
CarloSW2