The rivnuts shouldn't spin because you hopefully won't be tightening the screws very tight at all. Seriously, just a little more than hand tight, or you will crack the plexi. Also, I wouldn't put Loctite anywhere near the plexiglass; just the fumes will embrittle the plexi and it will crack. Since you can't tighten the screws enough to withstand vibration, and you can't use Loctite, I used a dab of Elmer's white glue on the threads and not a single one has vibrated loose in 750 hours.