For our Atlas Acadia 25, it came with a center flip-up center windshield and it really helps. We modified it so it goes completely horizontal for max air flow.
But if modifying yours is not an option, for a fan, we have never found any of the 12 volt fans to be useful. Not enough air flow. We bought two of those large size "Fan-Tastic" 12 volt box fans and they do push more air but are very very loud.
My suggestion is to get a small 110 volt inverter, maybe about 2500 watts and use a 110 volt house fan. They are going to be very quiet and will push lots more air than any of the smaller 12 volt fans. Be sure to check the air flow rating. By one, take it home and try it out for air flow. We have used a full size 20" box fan from an inverter while moored to keep cool, plus keep the bugs off in the past. Just size the inverter, plus 50% to match the needs of the fan.
Also, if you want to stay with 12 volt, consider a truck, RV or bus fan. They are usually chrome plated metal (you can paint them for the marine environment) but are meant to provide much more air flow than the typical marine fan. I've got one right at my pilot station on the boat. Lots of them on Amazon but be sure to read the reviews.
Bob/Bradenton