my bun did that too. just make it so that the coro is larger then the bottom of the cage, so as it goes about an inch out of the bottom. for shelves i made the coro the exact size and then took rubber hosing and cut a slit in it, slid in the coro, then taped/glued the top edge to the coro, leaving the bottom so that the shelf can be slid in too. then i put a small hole into the coro at the appropriate spots and zip-tied it to the shelf. the hose makes it a little harder to chew it up and move the coro off of the shelves.