In my motor home / caravan there is a passive ventilation in the bathroom and kitchen cupboard. Both are built to prevent rain coming in but small leaves etc sometimes come through. So I built this to just slide in from beneath to catch the dirt. 

When I printed it the first time, the legs were not quite in the right place so I snapped them off and printed little legs to glue on at the right place. Maybe the way forward for you if your cover is slightly different? 

Editable file included…