I also had a ferret who climbed into the box springs, tore up the stuffing,
and loved to sleep there.  I was afraid she would bring fleas into the
innards of the bed or get caught in the springs, so I got a fitted sheet and
put it on upside-down, over the box springs.  The elastic keeps it up and she
couldn't crawl in any more.  The sheet is literally fitted onto the bottom of
the box springs.  It works.  They will, naturally, find somewhere else to
climb into - like the couch cushions (very good destroyers of stitching).
[Posted in FML issue 1065]