This merits aero testing, because it makes a dramatic difference in airflow in the cabin:
I just did a top down 110 km round trip in 9C / 48F sunny weather, and was extremely comfortable with this setup ... dressed for it, and with the heat on the lowest fan setting. It made for a surprisingly warm, still bubble of air.
(It's a household AC unit.winter cover stretched over/ between the seatbacks.)
Without that wind block, I would have had to run the fan at the highest speed to fight a strong current of "outside" air flowing from back to front from behind the cockpit.