I did the same thing that Candy did.  I got a regular child gate and put
card board on it, but the whole way up (I have some really persistent,
wonderful climbers).  I used duck tape to hold the card board in place.
 
My WARNING is: After some persistent digging one of my girls, Tinkerbell,
got behind the cardboard ...  so at this point she was between the card
board and the plastic mesh.  She decided to try to go through the gate.
I would have never thought that even my smallest ferret would have fit
through there and tinkerbell's a medium size female.
 
Obviously . . . she got stuck, about half way through.  She was probably
there about three to five minutes.  I heard scratching but I thought it was
from her digging at the card board again.  It wasn't, it was her back legs
scratching at the card board trying to get the rest of the way through.  I
had a real hard time getting her out, it was a bit tense for a minute or
so.
 
My husband replaced the plastic mesh with plexiglass.  He did this by
first cutting the plastic mesh out of the gate ... then taking a pair of
needle nose pliers to pull the rest of the plastic out of the grooves (it's
stapled in there) ... then on each portion of the gate (each square) you
take the end or side piece off by tapping up on the top piece of wood and
down on the bottom.  There is are nails holding the end piece there.
Finish one square before you do the next or the whole gate will fall apart.
Once that's down, you just slide the plexiglass in and put the end back on
and nail it.  He said that it wasn't that easy to do, but I do love it now
that it's done.
[Posted in FML issue 3028]