I have a simular problem with the cats digging in the ferret's litter
boxes.  My solution was to use only a thin layer of litter in the boxes.
Ferrets don't cover up so the don't really need the deep fill that cats
need.  Just enough to suck up the wet.  We also have lots of boxes for the
ferret so any one box isn't really used too much.  Then change the litter
more often.  It is very unrewarding for the cats or ferret to dig in litter
that is only one stone thick.  The ferret does like to dig in the cats
litter box which needs to be filled deep, but that is a different problem!
 
Don
[Posted in FML issue 1253]