I just cut a thin plastic sheet from an old container like an ice cream bucket. Made it into the shape of the vent but a little wider with a tab to be able to hold on to, then cut out a 1/2" line towards the center, slide the sheet between the vent and holder and it blocks 98% of the air going through.
Loving wife and 2 beautiful daughters.
Chocolate Lab, Lily
2014 Dodge Ram Laramie Hemi 5.7 with 3.92
2014 Dutchmen Kodiak 290BHSL
ProPride 3P Hitch
Progressive Industries EMS-PT30C
Love my Weber Q1200
Me a Happy man on retirement!!!