Have never had anything come after my Ursack so no idea how long it would stand up to persistent little chewing things, but I definitely have seen photos of Ursacks where they simply drilled their way in, through the fabric.

For places like the Grand Canyon where it's a known issue, I'd want a metal mesh storage system, but for the casual explorer mouse, metal behind the top opening sounds prudent to me. Even a small piece of aluminum flashing would do the trick.

Cheers,