Thanks for the update!
I think that your solution is easier than what I am about to propose, but for those who want to keep the plastic ring, they might want to consider using something like
Loctite. From the description, the bolts are backing out from the threads due to vibration. The design didn't incorporate enough preload on the bolts, if I'm remembering my schooling correctly (but there's a good chance that I'm not

). The Loctite works as an added retainer to keep the bolts in place.