^ Agreed. I've fought a lot of fleece wars and no matter what I clip it with or weigh down the corners with, they find a way to chew through and get in. What I did was I doubled the fleece layer to let them get in between the layers on purpose--that is, I put my towels at the bottom to soak up pee, covered the pan with one layer of fleece, and then put one more layer on top of thatwith the corners cut out. Now they don't chew the fleece because they can get in between the two layers of fleece via the cut corners and sleep there. But my girls are easily placated, I think--as long as they can get under to sleep, they're fine. For some rats getting under it for sleeping is not the issue (they just like chewing it, I've had those personality types, too).
If they're giving you trouble with destroying fleece, you can always patch holes by hand with fleece scraps and needle and thread. Plus, places like Joann's or other sewing stores have lovely remnants bins, where they sell leftover fleece for really cheap, and when I'm not in the remnant bin, I'm at walmart, whose regular flat color fleece is about $3 per yard or so.
I switched to fleece to avoid the cost and mess of bedding, but really...sometimes the rats make it difficult to say that fleece is the better bargain when you're buying it all the time xD