I bought an Oakley ARO3 recently to hold my sunglasses. They have a groove cutout at the sides of the vents for a snug fit.

My Bell Z20 has rubberized grips in the vents to hold sunglasses, but they don't work at all for my Flak 2.0s. They're better at holding the wider Radar EVs, but I forgot while using the Flaks and chipped my lenses looking down a few too many times for my liking.