tango ur boat turned out awsome,,byw
with the larger /taller windshield the cockpit air becomes a vacuum as air travels over the shield the stagnet air in cockpit is being sucked out
air is now swirling from rear of hull of hull pulling in the motor fumes trying to find some feed air from the vacuum effect
I learned long ago from airplanes how the air travels and how to bend it and create swirls to create lift as the bent air creates a vacuum under the bent air stream
as why lots of the full wrap around windshield boats have vents in the cockpit to feed fresh air into the cockpit
now air is flowing in and out with the air over the shield so now no more swirling sucking in fumes from the rear,,,,
I see in ur pis small vents on the dash,,,where are u getting air from to feed them
if u add in some fresh air from the front deck the fumes should go away in the cockpit
my bolt on is just a windscreen/deflector just enough to blow wind over ur head,,,,not a full shield effect

The vents that you see are connected to a bilge blower that is located in the hull about 3' ahead of the dash. It really does not pull in any "new" air. I added those just to have a breeze in the no wake zones.

If you were going to try a deflector behind the rear passengers, how tall would be your guess as a starting point?