Do you have access to sheeps wool?? You could stuff that in the walls behind plywood. I know it would cost more but what about the spray foam that expands. if you were able to put that up, then cover with plywood or partical board so the birds couldn't get to it. Sometimes if you talk to places like Menards they have the cut outs from exterior doors (where the window goes) you can get them and line the walls of the coop with them. I know a few people who collect them and put around the foundation of there own house to help in the winter.
Just a few ideas. Probably not much help, I will try and think of better ones...
Just a few ideas. Probably not much help, I will try and think of better ones...