Cardboard = bad. Mold grows on that in a day without a problem.
There is absolutely no such thing as an enclosure that is too big. There is only such thing as a hide box that is too big. When a BP is safe and secure in a hidebox, it is completely and totally oblivious to anything outside of its hide. But, when night falls and it wants to cruise around in the dark, that's when too SMALL becomes an issue. You're doing just fine, beaglegod.