There are a few different ways to deal with this problem.
The Brick Method
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.
Since I started building C&C cages, I’ve discovered there are two things I can’t do without: zip-ties and binder clips. I have two different sized binder clips on hand and I am surprised (and amused) at how many are in use (14 in one cage and several more for their pen). Binder clips are a wonderfully effective way to stop burrowing. It requires cutting the fleece large enough to run up the sides of the cage so you can binder clip it to the top edge of the coroplast. I cut mine an inch or two longer than the sides, so I can fold it over the edge before clipping it into place. Not only does this prevent burrowing, it stymies coroplast chewing (in case your guinea pig has a penchant for scalloped edges).
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.PVC Pipe Frame for Fleece
Another method to keep fleece down is by using a PVC pipe frame. It’s not a common solution, but I’ve seen descriptions of this design once or twice. The idea is to make a frame of 1/2 inch pipe that is just slightly smaller than the inside edge of the frame. The fleece needs to be a few inches larger than the bottom of the cage. The fleece is placed over the frame, the edges tucked under, and the whole thing is placed into the cage. It eliminates any raw edges for the guinea pig to burrow under. The weight of the pig on both the PVC and fleece should hinder them from tunneling underneath.
I recently discovered a variant of the PVC frame technique: the pipe frame is replaced with a sheet of coroplast that is a little smaller than the bottom of the cage. The coroplast is covered with the fleece and binder-clipped into place, then placed in the bottom of the cage. This would be a great way to use up any large chunks of coroplast you may have laying around. It might be easier to clean than the PVC frame, too.
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.