Disclaimer: This post was originally posted by Hero Member, PaulB. This user is located in Southeast New Mexico, Zone 7. This was originally posted on June 19, 2014.
1/4 inch or 1/2 inch mesh hardware cloth (wire). They can't wiggle through it, bite through it, and you can cut and stich it up with thin wire into a box shape that will fit right over your box. It comes in a variety of widths and is readily available at any building supply store. Do NOT use chicken wire, they can crawl right through it.