I can’t pull it snug bc then it would be a fire hazard. I just need it to stop flapping when the dryer isn’t on .

I think there are some with vents in them

There are other styles of exhaust hoods that are less likely to rattle in a wind. I have this one. It should be painted (typically to blend with the brick or siding or whatever) to protect it from UV deterioration.

Heartland Dryer Vent Hood

This is for a noisy vent hood but it should work for a dryer vent.

Maybe the flat closure spring /tension is not working? Remove and Replace the vent or closure mechanism.