I unfortunately don't have an answer of how to ferret-proof a couch.  But
I do know that it is VERY IMPORTANT to keep them out.  In December my
youngest ferret Callie tried to climb into my couch, but she got stuck
about halfway in and sufficated.  By the time I found her it was too late.
So, if anyone out there has any suggestions of how you can ferret-proof
couches, I too would be interested in hearing.  For now, I just try to be
sure my other ferret (Marti) isn't left alone in the livingroom.
 
Good Luck!
Jenn
[Posted in FML issue 1152]