We have a beagle and two Chihuahua mix dogs, plus we very the coast/beach often. Dog hair and sand easily get past closed louvers. So what works for us is to wrap the vent in a couple of layers of the Blue painters tape. You can tape just the underside of the vent and the tape will trap ALL dirt from falling into the vent. It is easily peeled off for winter use.
2011 8314 BSS Rockwood Signature Ultralite
2005 Dodge Ram 2500 Quad-cab SLT