Some power cachers just have stickers and stick them on the outside of the film can.
Or, do they like they are doing on the Alien Highway series. Bring along some film cans with logs already signed in them, and just replace the caches as you go.
Really, this part of this discussion deserves it's own thread. There is a fine line between what is a new aspect of the game and what would be more suitable for a new, similar game.