They can still probably get through the grille on the lid.
I have had one infestation in my shroud and when I was building my engine I found mouse poison on top of the pistons.
I installed the screen mostly to prevent road debris from going into the fan while driving.
I made two D-shaped aluminum pieces and sandwiched the screen between. Then I pop-riveted that to the driver's side of the firewall.
I can send the pattern for the D pieces to anyone who wants it. It is for a CMC. I don't know it the cutout is the same on other flavors.