If you are still looking for possible solutions to stop critters from getting in through the louvers you might consider expanded metal. Lowes & Home Depot have it, usually with glass & plexiglas - near where they sell doors. It has a diamond pattern made by a machine that makes slits in sheet aluminum then "stretching" the plate to open the slits into a diamond shape. This stuff is often used to protect screen in screen doors from being pushed out of the frame. (Heavier gauge steel is sometimes used in BBQ grilles to hold the charcoal).
Anyway, making a wooden frame to go around the louver then mounting this to the opening should deter the critters (be sure the frame stands out far enough so the louvers open). If screening is used to keep bugs out I would suggest it go on the inside the of the expanded metal for protection.
I have used this material on other projects to deter squirrels and it works very well. Once I build my GH in a couple months I hope to do the same thing, except perhaps using aluminum angle rather than wood. I am considering doing the same thing for the roof vent.