I found oversized cardstock tags at an office supply store. The tags already had strings on them so they were easy to attach to the cat carriers. I cut different ribbons to size and glued them onto the tags, then wrote each cat's name with a Sharpie (Kit & Kat always rode together). On the back I included our address and phone. They could be laminated at a copy shop for longer use.
Not once did we come home with a dog named Spot.