Try some vanilla extract on cotton balls - works in an antique dresser I store linens in, but I do have to resoak the cotton balls every few years. Also, try some clay kitty litter (non-scented) if you think you can vacuum it out of the case again. Otherwise, fill a large plastic bag with kitty litter and set the case on top. Close the bag tightly and leave for two weeks. This always worked for me when I was a librarian and people brought back skunky books!