The cabin air filter is really just a screen to protect the fan from leaves, twigs, a mouse nest and other matter – and really has no filtering effect in reference to cabin air quality. And a screen at the cowl opening would hopefully keep a mouse from creating a nest in there.
What is labelled as a side cowl grille, is in reality, a bezel desperately in need of a metal screen mesh to keep critters, leaves, twigs, etc. from finding their way into the cabin air blower fan, unless you are quite confident that your car is kept in a clean, critter-free garage, when not being driven. I doubt that the one integral strip of plastic serves as a beneficial barrier against anything other than perhaps your fingers.
Goaterguy was probably quite surprised to find a mouse nest in his, and added a metal mesh screen to prevent that from ever happening again (they chew through plastic, so don’t go through all the work of adding a screen made of plastic). I know from experience in a camping trailer, that the urine smell never goes away, but hopefully his mice that spent the winter left the nest (and vehicle) for that activity.
And since the air-flow needs to be unrestricted, I would suggest also avoiding the installation of any “filter” meshes, like window screen, as it will clog with dirt just like the air filters for your engine and furnace, and become another maintenance task (will need to be checked and cleaned periodically – not ideal).
In anticipation of taking on this project, I ordered a sample of gutter guard from Houzz.com ($2.75), but you could probably procure a 3′ panel for around $5 at your local hardware store. Since my car is kept in a garage free of critters (thanks to a tightly sealed garage door and an ultrasonic pest repeller, after lifting the edge of the passenger side cowl panel to see everything looking like a new car, I gave up proceeding with this project. My understanding is that , in addition to the interior A-pillar and header panels, and the exterior windshield surround panels, the cowl grille panels too, need to be removed when replacing the windshield.
It is not easy removing these panels though* !!!
But for those that might wish to proceed (Steve Southwell advises avoiding putting much pressure on the lower overhanging edge of the windshield)…
* COWL GRILLE PANELS REMOVAL/INSTALLATION
1. Remove both wiper arm assemblies (note which is left and right and their orientation, so that you can later replicate their position on the splines).
2. Disconnect washer hose B from joint pipe A (twisting back and forth wile pulling).
3. Detach the weatherstrip clips A and B (what a PIA; actually it might be best to stretch the rubber to separate the clips from the weatherstrip, leaving them in place on the metal body).
4. Remove the detached weatherstrip.
5. Remove the hole covers.
6. Remove the (phillip head) screws.
7. Pull the cowl grille in the direction of the arrows shown in the figure and detach the cowl grille hooks from the body.
8. Pull the cowl grille in the direction of the arrows shown in the figure and remove the guides from the windshield.
9. Detach windshield washer hose B from the cowl grille hook.
10. Move the cowl grille (RH) in the direction of the arrow shown in the figure.
11. Move the cowl grille (LH) in the direction of arrow (1) shown in the figure.
12. Remove both cowl grilles (LH and RH) in the direction of arrows (2) as a single unit.
13. Remove the side cowl grille carefully so that you can reuse the foam gasket.
14. Trim the screen mesh to fit on the underside of the grille, and fasten with hot-melt glue and a couple of small tie-straps.
15. Reinstall everything in the reverse order of removal.