Of course, our litter box is in a corner.  (Wouldn't a cylindrical cage
just drive a ferret up a tree???)
 
We use two identical pans.  One has holes punched in diagonally opposite
corners, and is the box that is actually restrained.  Through each corner
is one half of the type of key ring that has a ring on each end, and can
separate in the middle, when pushed together (does this make any sense?),
the other half of each key ring is around a joint in the 1/2 inch spaced
cage wire.  The box with the litter in it sits in the first box, which is
the one actually restrained.  Makes taking the litter-filled pan out easy
(for scooping), and it is also reasonably easy to get out the whole thing
for major cage cleanings (but not so easy I'd want to do it every time I
want to take care of litter).  I'll try an ASCII graphic or two.
  _    _
 <_>--<_>  key ring  two rings with a joint that also acts as a swivel
 
 _____________________
|________/<-restraining key ring
||      |
||boxes |
||      |
||______|
|/<-restraining key ring
|
|
 
Liana, Totoro, Neko, and Akira
[Posted in FML issue 1150]