I have used plastic gears and zoom rings on a couple different kits, all with Aquatica housings. On a couple occasions, I've had them slip to the point that I could not make an adjustment on the dive. The ring would become cocked on the lens and not align properly with the operating gear. Probably from turning the gear too hard into it's stop.
I solved the problem by installing small SS set screws at three points, 120 degrees apart, on the ring. Once I snugged them up against the lens, problem solved. Just be careful not to tighten them too much to damage the lens barrel. I found that the O-rings on the inside of the zoom rings don't do a good enough job holding the ring in place.