If you are installing a vent and/or fan, why not cut a small piece of bug screen mesh to fit? Do it to all your cabs little openings, speakers too. This will keep most of the little bugs out of your cabs.
Cut it just a bit smaller than your vent/fan/speaker cover and mount it in behind (cut the mesh so that the cover screws go through it); or in the space invader cutout idea, put the mesh on the inside. The mesh is made of nylon mostly these days and is easy to cut with scissors.
I just used some mesh left-over from a door screening project, but you can buy it very cheaply by the metre at your hardware store. Use a light grey colour if possible (less obtrusive/obvious for most situations). I can't even see the mesh unless I look very carefully.
This really helps me as I am in a hot humid climate with lots of little bugs and also little lizards (geckos mostly, and larger too) that just love to go inside and lay eggs, sh!t, get electrocuted, and generally ruin my cabs.