|The brick method|
There are a few different ways to deal with this problem.
An easy, quick and inexpensive way to keep pigs from burrowing under the fleece is to hold the edges down with something heavy. Bricks, a big food bowl, hidey houses or anything hard to move or get under can do the trick. I have a brick under my girls' water bottle. It works double duty: it keeps the excess water from soaking the fleece as well as keeping the fleece from being pulled up. The drawback of bricks is that they take up room on the floor and can be obstacles in an otherwise open running space. If you have truly tenacious pigs, any open edge is an invitation to get under the fleece, so the heavy object approach may not be practical.
I have heard of a variation of the binder-clip solution by sewing velcro to the fleece and sticking the matching side of the velcro to the coroplast. Personally, I'd think that velcro would be hard to clean around and lose its grip too quickly. However, it's worth mentioning - you may come up with some nifty way of making it work.
|1/2" PVC pipe frame|
|PVC pipe threshold between shavings and fleece|
These are the most common prevention tactics I've seen. I'm sure there are other methods that people have dreamed up in order to thwart their industrious guinea pigs. If you're lucky, your pigs have never considered that burrowing under their fleece bedding is a possibility. Keep your fingers crossed that they don't become enlightened.