How about lining it with "coated" plywood panels, the type used for concrete shuttering. Should give him nothing to get hold of but if their is, cover with steel strips that can be angled at any metal works factory cheap enough. I had some made not long ago from a company that makes steel gutters for industrial roofs. Looking at the weather we are expecting over winter it should help with the insulation. All the best mate, it sounds frustrating.