What callsign should an ultralight use?
One of the suggested communication strategies for uncontrolled airports is to announce the type of aircraft and color. e.g. “Brown and white Cherokee entering downwind”. 

Rather than using your initials, which like the tail number, don‘t really give any information, why not use your color or some distinguishing feature. e.g “Red tricycle ultralight” or “Blue weight-shift ultralight”. 

Anyone out there would have a better chance of knowing what to look for and seeing and avoiding you.