Biff has a plastic tray in the bottom of his cage - is this what Milly has scratched, or does her cage not have a bottom in it
How is she when you come home? Does she look as if she's been stressing?
(Biff is usually still giving it

when we come in

)
If she's going in on her own for a nap, I think she likes it, so it's probably not the crate that's the issue, but the fact that she's been left alone.When she was uncrated, she tried to destroy the house, so now she's shifted her energies and trying to destroy the crate
Is she off leash on her walks? Boxers really need to have a great big run around off leash, or another dog to play with, to tire them out - a tired boxer is a happy boxer!