I have never had a problem with my doorway ferret barriers.  I use a piece of
plexiglass, cut to around knee height.  I run a groove down the middle of
1" by 1/2" wood strips to make a track, but if you don't have woodworking
equipment, you can just cut the plexiglass to slightly wider than the
doorway, so the tension will keep it in place.  It's easy to step over and
can be removed when not needed.  If you've got a jumper, a piece of garden
hose, PVC or plastic tubing can be split down the center and glued over the
top of the barrier.  This gives the top edge a rounded surface so they
can't get a good grip when they jump for it.  I also put a strip of
dark tape on the clear surface to deter them from running into it.  The
clear plastic prevents anyone from getting stepped on by a large black lab
friend whom we allow to jump the barriers.
 
Lynn Burroughs
Magill (the demon beast)
Guinness (the neurotic sissy-boy)
[Posted in FML issue 0979]