I used to do this in my old house...my husband and I are "home
improvement impaired," meaning we can't build things.  Anything more
difficult than a pre-made bookshelf, and we just fall apart.
 
I simply took a piece of plexiglass slightly larger than the doorway,
and bent it (gently!  Slightly!) so that it fit.  When I let go, the fit
was snug.  REMINDER -- put some kind of colored electrical tape on the
outside of it (away from the fuzzies) so you remember not to try to walk
through it.
 
There is a simple knife available, you just score your sheet of
plexiglass and snap off the excess.  I needed mine extra tall (one of my
ferrets can fly), so I kept it at the full height (I'm very tall, so this
wasn't a problem for me.  Everyone else in the house was miserable,
though :-)
 
One other word of caution.  If the floor between the two rooms is uneven,
there's a chance the ferrets can push the plexiglass up from the bottom.
One of my ferrets is a Houdini, can escape from anywhere if he's not
watched like a hawk (pardon the expression!).  So our room had linoleum,
but the next room had carpet, making it slightly higher...originally I
had the plexiglass bowing outward, but they ganged up on it and pushed it
up.  Then I bowed in inward, and the little buggers figured that out,
too, after a while.  The problem was that the little strip of metal that
divides the carpet from the linoleum caused a little teeny gap between
the linoleum and the bottom of the plexiglass -- literally, only a couple
of millimeters -- and those little monsters figured out how to lie on
their backs (all 4 of them worked together on this), hook their claws
under the plexiglass, and push it up.  I've never heard of anyone else's
ferrets doing this (although surely there are many out there), most
people are very happy with the plexiglass.
 
Take care,
Chana
[Posted in FML issue 4301]