installing screen under Duster's air vent

Thanks to everyone for the ideas. I've attached two pictures - one showing that I've cut a large access hole in the firewall (left side and right side) so I could inspect and repair rust-thru damage from trapped leaves. I used a pneumatic chisel to shear rivets that held the two air cowls in place. They were too damaged to salvage, so I had a local shop make two new ones. As for installing the screen under the vents - I will use these two holes to slide in the screen from one end or the other. I'm guessing that it will be a tight squeeze to slide in a screen that is big enough to span the vents - so brackets will become problematic.

My idea to use foam saturated with fiberglass came from previous experience when I used foam to apply clear polyurethane to wood. When the polyurethane dried - the foam was hard as a rock. If fiberglass resin dissolves the foam, I'll use polyurethane or something similar.

Fixing the holes in the firewall will be a project in itself due to the odd shape - but it necessary to repair rust and stop future leaves. Here in Florida - I get falling leaves and junk year round.

I could be wrong, but I'm pretty sure that the factory had a hunk of foam on each side of the vents - perhaps to divert air and prevent rain from blowing over the cowl and into the vent box at high speed.
tx,
Ken