New regulations in many states do not allow you to use hanging placards or "temporary" registration plates. They must be permanently attached to the boat. There is a long model number on your transom, write it down, call an inflatable dealer, and they will be able to tell you if it is pvc or hypalon. You need to know to get the right glue. There is a kit you can order your reg number on for about $35. It looks nice and glues right to the boat. Permanent and professional. Paint peels quickly after a year and looks terrible.
If the hull is not black, you can use marking pen and a stencil. It's easily renewed. Have to say I've only tried in on a PVC boat, but it worked great.