You can just get a vent sized sheet of mesh and tape it up there with the heavy duty tape of your choice. I did it at the last house, and I think it only cost about 50 cents.
Just make sure it's empty before you put it up... a neighbor at my last place did it not knowing there were some babies inside and between the hungry babies crying and the mother going frantic the whole neighborhood was full of noise all afternoon... and then my neighbor felt so guilty he gave up and let them live there after all
