Here's an update: Charlie hasn't dug another burrow. He spends winter nights in his heated night box, but he refuses to use it once the weather gets mild. Our summer lows are very warm, in the 80s or high 70s, so he really doesn't need it then. He didn't use his other house much when it was new but now he spends every night and part of the day there during the summer. We call it his summer house. He emerges from it with dirt on his back and the dirt inside is very soft so I guess he digs a little to get comfortable, but no burrow. I think he likes having protection on 3 sides and on top since he always naturally headed for corners to sleep.