You have a few options -- all of which include using a different method to feed hay. There really isn't anything you can put on the plastic to stop him from chewing it.
You can see photos of some alternatives on this page of my site:
http://rabbitsindoors.weebly.com/hay.html
However, the easiest way (other than a hay rack) to offer hay is to place it directly in the litter box. I realize that many are turned off to this idea -- I was too when it was recommended to me -- but it really is a great, easy, way to feed hay.
Rabbits won't eat soiled hay, so don't worry about that. The idea is to refresh the hay twice per day. They naturally like to potty while they are grazing. By refreshing twice a day it ensures they always have fresh hay, encourages more hay-eating (very good!) and keeps the litter tray odor-free for a longer time.
It was long-time experienced rabbit owners that first recommended this method. I resisted for quite awhile. But I've been feeding hay this way for about 10 years now and wouldn't do it any other way.