I'm an amateur radio operator, also referred to as a "ham". I have a federally licensed call sign, and my state's licensing bureau would produce a license plate for me with my call sign on it. Last I checked, they charge an extra $25 for this privilege every year that I renew the license plates, even though they don't give me a fresh plate every year. On previous vehicles, I buy stick on letters & numerals, black print on white highly reflective background, at the hardware store to match my call sign, and apply those across the bumper to indicate my radio license number. This makes the bumper stand out in a parking lot from a great distance away, and is also highly reflective, which is helpful in some environments. My Carnival is white, so the black letters on white match the paint scheme.
My state used to require front and back license plates, but now they only issue rear license plates. For my new Carnival I will buy an extra set of letters to put on the front bumper in addition to those on the rear.