My northern pine loves to burrow. Right now I've got him on aspen, and he loves to dig through it, but he likes to dig a lot around the edges and this pushed the aspen up into piles in the middle and leaves the floor bare around the edges. Then he tries to dig through the floor , lol. So, short of just adding more and more aspen, what other options would be good for him. I want him to be happy, but I also want him to be safe, and I want something easy to clean, any ideas?