Hi June,
I do not have cats, however I have seen covered, ferret litter pans that
would appear to be too small for a cat to fit into.  I believe I saw them
at the Ferret Store.  This would probably discourage your cats from trying
to use your ferrets boxes.  It will mean buying new litter boxes, but it
might be worth it if you are really have that hard of a time with it.  But
you could just use the boxes you have now as clean reserves for when you
change the ones in their cage.  Good luck!
 
Jenna
[Posted in FML issue 2929]