If it gets enough light/sun that plants grow, put a peppermint plant out there.
Rodents hate the smell.
If nothing grows there. You could spray peppermint every month, or hook up some kind of rope/ladder for them to get out. If you go the "escape route" way tho, they're gonna live in there having babies and you won't be able to open the window or they get into the building.
Might be better to put a finer mesh over the chicken wire.