The question is in the title. I would like to do something for the Fanart competition.
What comes into my mind are the skulls of the woodlanders (like Cluny's mouse skull as a clasp on his tunic).
This is a bit ironic, since they used Redwall's tapestry as a banner for a short amount of time, but I think a grey cloth with an animal skill of some sort would do.
Considering Cluny's trademark weapon, his tail, if it were to have a real symbol it would likely have a whip or a lash. In the book, however, the most symbolic thing was the skull on the pike, though if I remember it wasn't a mouse skull.
Cluny's standard was the ferret skull on a pike like Skarzs said, but I dunno about any banners. They were never mentioned in the book. My guess would be a black or grey field with a red ferret skull.
Mole skill. ;)
You've said "skill" instead of "skull" twice now. :P
:-/