I have a solution that has worked well for me.  I measured the inside of
my window opening.  I then went to a store that specializes in the wire
shelving for closets and had shelving cut to that size.  It fit into the
window opening and my girls were unable to get to the screen to scratch it
or climb it.  To hold it into place you can let the window down to just
below the top of the shelving.  Because the wire goes vertically they are
unable to climb it.  I haven't had a ferret get to a screen since.  Hope
this helps.
 
Ann and crew
[Posted in FML issue 3460]