It is for a traveling production, so it needs to last for a while. The gaff idea sounds good though.How permanently?
I mostly work on theater shows with a run of about one month, which means tagging them permanently isn't really necessary. For longer runs, you might want to consider e-tape or gaff tape around the yoke or plug.
Some television studios and theme parks I've worked with will use white paint pen on the body of the fixture, large enough to be legible from the studio floor. That wouldn't ride in most theaters, though.
Yoke label next to the c-clamp? Same location on every fixture.