I went to Wal Mart and bought the regular "baby" gate.  It's got two
frames that telescope in and out by means of a locking arm.  It's
adjustable from 24" to 53" wide and easily adjusts to any opening.  It's
like $9 and it has a plastic mesh in each frame.  To keep the ferrets from
climbing up the mesh, just take the box it comes in and cut out two pieces
of card board (1/8" to 1/4" thick Plexiglas will also work but will cost
more) that just fit in the frame recesses and cover over each mesh on the
side the ferrets will be.  Secure the cardboard covers into the frame with
a strip of duct tape around the four edges of each frame.  In only about
20 to 25 min you have a fully adjustable ferret gate with rubber edges
that will fit almost any opening for less than $10.  And if you turn the
brown inside of the box cut-out to face out of the mesh, it doesn't look
bad because the "brown" of the box almost matches the color of the frame.
I've used two gates like these for over two years and the only way the
ferrets have gotten past them is if a chair was too close to the gate and
they used it as a launch pad to fly over.
 
Hope this helps.
 
-Weezzer
[Posted in FML issue 3806]