We either put small holes in the litter box and used wire to keep it still.
Then we resorted to the heavy clamps on the wire and box.  This seems to
keep the box in one place.  I've heard litter can make a ferret dig at
their box more.  We use wood stove pellets which work great.  None of
ours will dig the litter out.
 
You might get a big rubbermaid container and put in 5 pounds or so of long
cooking rice.  This will give them something new to dig at.
 
Debbie
[Posted in FML issue 2858]