I recently bought a new FN 182. I took the plastic pans and covered them in fleece, kind of making a pillow case type cover for them. The problem is that both of my chins are major chewers and keep pulling the covers off. The one burrows under the liner and chewed a hole in the pan in the middle of the night. Is it safe to put a zipper on the liner? Im running out of ideas and I don't want to waste the liners I already have.