Take a sharpie and trace a line around the inside of the return plastic trim. Remove the trim and cut off the excess blocking the return vent. Tape it sealed.
Inside the return duct, one side should be open for the return air to go through the cooling coils.
Seal everything else. I could put three fingers through this hole. This allowed hot attic air to get into the return duct (before and after pictures).
As for the vents, remove the vent, trim the flange (neck) that protrudes up into the duct so it's flush with the styrofoam insulation inside the duct.
All of this will help improve airflow.
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