As we all know now, the cheap plastic air vents supplied by van's kits leaks air bad. After stuffing the inlets with plastic bags, then rubber banding cellophane on the inside, this year I found a good solution...besides replacing them will aluminum retrofits.
Prescott_Plastics_LLC sells 1 3/4" Black Vinyl Round End Cap 1.75 Rubber Cover Pipe Tubing Stopper that fits perfectly. Can install and remove by hand. After a test flight, they stayed in place.
Found mine on ebay. 4 caps for $7 including shipping.