How do i get the fleece to stay put in my rat cage?

The top level of my rat cage has a wire floor so i cut fleece to fit the top level and made ties in all the corners. How ever my smart little rats dont chew through the ties, they figure out how to untie them all and they bring the fleece down stairs. They have lots of other bedding in the cage so idk why they do this. Anyways is there any ideas on how i can get it to stay put? I dont wan to retie it every morning, and i dont want my babies getting bumble foot. Please help!