had a fam of flying squirrels in a knee wall 2 years ago, they got in through a hole in the soffit. I screwed a one way door thing to the hole and left it for awhile. Boy did they scrape the hell out of the flashing around the one way door trying to get back in!

Eventually I had to cut open the knee wall from the inside to clean it out and re-insulate. It was a mess but no dead squirrels were trapped inside.