Our cat doesn't get into boxes, no matter how comfortably lined. Cardboard boxes are for deconstructive art. My guitar case, however, is quite another thing, but unluckily for her it isn't often in use these days as the guitar tends to be in a gig bag most of the time and that has no appeal whatsoever.
We have found that anything bought or made especially for the cat (such as one of those furry cat shelter things or the cat scratching post) is totally ignored.