Hello , you might want to place panels of 1/16th of an inch thick
Plexiglas from the floor to about 24 inches high along the balusters from
one end to the other with no gaps.  The idea is to create a clear wall
the the ferrets can't climb over plus you won't have some unsightly
barrier.  The "plex" is ideal as at 24 inches the fuzzies can't get over
the top and just slide off .  If this seems too low then take the barrier
up to the bottom edge of the rail.  Use velcro to attach the panels this
way you can take them off for cleaning .
 
As for heights , we once lived in a high-rise in Baltimore on the 8th
floor, we were ferret sitting a big guy named samson.  We went through
the apartment and ferret proofed (ferret proofed is an oxymoron as a
ferret will find a way ) all the dangers but over looked one , our window
screens .  The screens were metal and heavier gagged than regular metal
screens .
 
At night I cracked the window open about two inches in the bedroom , when
we awoke I glanced at the corner of the window and there he was , just
sitting there on a 8 inch ledge catching the morning sun , he obviously
was aware of the heights but was so happy watching the morning traffic .
I went to the window slowly so as not to scare him , there in the screen
he had tore the screening out of one corner , I pulled the screen off and
hung outside , samson looked over with a look like " oh , you're awake ,
what's for breakfast, he just cruised over oh so casually .  I really
scared the heck out of me , we put metal lath up and he didn't even try
again .
 
Ferrets are great teachers if we only listen ,,,,ferreterr
[Posted in FML issue 3975]