I think I would use tacky glue to attach a loop of pretty ribbon to the back. If you want an "invisible" way to attach it, run some yarn or ribbon through the strings on the back so that it catches about an inch worth of strings. (It would look like a sideways belt-loop.) Then you've got a loop that can catch on to a 3M hook. Or since the letter is made of paper-mache (sp?), you could probably cut a hole in the back of it before you wrap the yarn around, and a nail or hook could slide through the yarn into the hole.