This. I had a guinea pig once that startled me one day by having an extremely loud fit. I found her stuck inside her cardboard tube panicking because she couldn't get out. Clutter is usually a good thing with BPs, and with my smaller tubs I've taken to wadding up paper towels (what I use as substrate) to take up most of the negative space so they can hide under and around it. They'll wad it up or hide under it anyway, may as well give them some extra to work with.