WTG jeep and welcome tp the Hobie Forum.
Been there, done that, but I used a cutoff piece of pool cue stick that dropped right in the mast holder until it stuck. Drilled out a hole for the Wal-Mart bike flag pole. Worked great until I tried fishing with it--then it got in the way of my casting, and I haven't gotten back to using it again. I found a Florida State flag at Boat USA that clipped right to the W-M flag pole, and was the right size. But I like your idea just as well--whatever works is my motto.