I found a length of plastic gutter leaf guard at Home Depot, it looks like a plastic version of expanded sheetmetal, like you would find on a porch screen door. 1/4 inch or smaller holes. I cut this into a rectangular shape that would tuck under the end slots on each side of the cowl vent. Then I used a black zip tie in the center to make sure it won't fly away. A bit of heat with a heat gun made it lie down flat. It's not super noticeable installed and it might eventually scrub the paint from movement, but I got no more leaves in my vents!