I know what you mean, but when I opened the DV window it wasn;t a howling gale at 140knts.
And the difference between a bucket and a flat plate is pretty much negligible when put through water. BUt its slightly different because water is incompressible.
The air molecules will go to the area of least pressure which will be over the roof and round the sides when the cabin becomes pressurised with the dynamic air flow.
If the rear window popped out the pressure would then drop and there would be more flow through the cabin and more drag.
But I stand to be corrected.