Make your own out of a small similar shaped piece of stiff plastic. Put it on the bottom where you can get a straight shot to drill a pilot hole for the screw that holds it. Dab a bit of silicone in the screw hole, tighten just enough.
Went camping once in a decent windstorm. We had loosened the two tabs on either side to let the vent work. Middle of the night that cover started flapping in the wind something fierce, making a heck of a racket. Nothing like fumbling around in the dark in a windstorm trying to figure out whats making all the noise.