I found this one which is kind of neat. Once you understand the basic principle, you can adjust the size to any mug. This would be a really cute teacher's gift.
https://drive.google.com/file/d/0ByH...zJi/view?pli=1
After reading thru the directions again, I would omit the pockets inside the cup. I think that just fills the cup with fabric. But I do think I will try one just to see if it works.