One cheap and easy idea that WILL work is to get some 1/4" galvanised hardware cloth, cut it a little bit larger than the opening, and stuff it up inside an inch or so from the bottom. Next, drill a small (1/4") hole an inch or two above the screen, get a can of great stuff, and squirt some in. Put enough in so that it will expand and come out the bottom, and cut off the excess when dry. Plug the hole with something that looks good, or even use a large pop rivet or screw and paint to match. Mice, squirrels and bugs will not be able to get through this. It is cheap, quick, easy and it works- the kind of project we all like! Oh yeah, you will need a way to hold the hardware cloth in place until the foam stops expanding, keep that in mind.