I don't know how much clearance there is between the fins and the shroud as a restriction (some airflow across the core has to turn to exit through the fan assembly), but you could add vent holes to the corners and add flaps. They can be made out of thin, flexible rubber, overlap the holes all the way around behind the shroud, and are hung with washers, nyloc nuts, and two machine screws (#8 would be fine or rivets are an option) side by side and on one side. The flaps operate on differential pressure. At low speed, the pressure in front of the shroud is lower than behind it from the fan operation, so the flaps are closed. At higher speed, the pressure in front of the shroud is higher than behind it, so they open, increasing airflow through the core.