I have found that Marshall Ferret Bi-Odor Waste Deodorizer that you
add to the ferrets water works great.  It eliminates the odor before
it starts.  I have used it when I have had as many as 6 ferrets.  I now
have 3 and the cat smells worse than they ever did.  I purchase it in a
32 oz. bottle and it lasts for a long time even adding it every day.
[Posted in FML issue 4752]