Do you only supply hay in the morning and evening? You really should leave them hay 24 hours a day, not just give them a pile in a morning, and another pile of an evening. She could be bored through simply not having enough hay to eat which is why she has taken to chewing the plastic, so a simple suggestion is, put loads of hay in for them to play with, eat, and nest in (even if they are neutered they will still nest) and see what happens. You need to break the habit of chewing plastic as soon as possible, because even with lots of hay to eat if she is of the mindset of chewing plastic now, that's it, done and dusted.
Our girls have a foot thick of hay in both their shed and run, permanently, plus a hay rack in their hutch which is in the shed. We just add fresh hay to it when it starts to look a bit tatty - I put some in this morning, oddly enough. Adding hay on top of "old" hay just makes the older hay an insulation layer really, if they're outside rabbits, plus it helps protect their hocks against becoming sore.
But to be honest, it sounds like your bun is bored and not having enough hay so my advice would be to give them permanent access to hay, and not just give them some morning and night
.