First off let me say I do not believe there is such a thing as ferret
proofing... but you can make things very ferret resistant.  Here's what
I did with some of my screens:
 
Small screws into the screen frames will secure the frames into the
window so they can not be pushed out.  From Home Depot, Lowes or another
hardware store, you can buy 'hardware cloth' and prefabricated screen
frames, or if you can run a saw, you can make your own frames.  Hardware
cloth is basically the same as the wire we see on many cage floors, and
comes 1/4 x 1/4 or 1/2 x 1/2 inch.  Either size will work.  Cut the
hardware cloth to fit just inside the window box (that's the hole in the
wall your window sits in) putting it over your window screen.  Place the
second frame you bought or made, over both and affix the second, outer
most frame, securely - around the window frame.  When you get it all
together, go in the house, open the window and push on the screen and the
hardware cloth now backing it.  If you did everything right, it will only
give slightly in the middle.  There will be no give around the outside
edges where the ferrets could weasel through and even if they demolish
the bug screen, the hardware cloth - being basically cage wire - will
hold them.  You can spray paint the wire before you put it up to match
the house, or screen colour, or what ever colour.  You can also cut the
hardware cloth a little bigger than the entire window, use a frame a bit
bigger than the window box, and attach both to the exterior wall.  Just
keep in mind that your ferret is not going to go through the window, but
even in an emergency, you probably aren't either.  Prepare accordingly.
[Posted in FML issue 4498]