Here is what I did:
1) Cut a piece of 1" plywood to the same width of the door by 24".
 
2) Put on the bottom and sides something to protect the frame of your door.
I use these strips made of rug with adhesive backs, but I don't trust the
adhesive, I nail them to the barrier.
 
3) Insert between door and door-frame.  You can see over it, you can (with
a bit of difficulty) step over it and hopefully the ferrets cannot get over
it (make sure there is nothing to climb near the door).
 
If the ferrets get over this then make a new piece 30" high and try again.
 
Hope this helps,
Joe and the gang of Two (Slinky, Pale Face)
[Posted in FML issue 2624]