Hi petek,
3/16" Lexan should work fine.
As to the direction of the pressure, if you observe carefully, the wind that buffets you in your Cobra's cockpit comes from behind your head!
The airflow pattern at highway speed is over the top of the windshield all right, but then it curls back around and hits the back of your head. So the purpose of the windstop is to block airflow coming from the rear!
As to the buffeting, it is certainly real for me - that's exactly why I made the windstop. Dunno if it works yet - waiting on weather to clear enough to allow a test
Tom