When I first got my babies the cage I bought came with a high back corner
litter pan.  It was big enough for them to put all four feet in...of
course they were just babies and small.  Well, they did alright for a
couple weeks, then they decided they didn't like it..  then I bought a
triangular corner box, even worse, they didn't fit quite right in it when
wanting to poop.  Finally I tried a rectangular, low front, litter box,
with a plastic removable rim that prevents the litter, mostly, from
flying out when they feel like playing in it.
 
That was the trick...they moved it around in their cage until it was
where they wanted it, and they almost never miss, unless it's not clean
enough.  I put one in their play area and about 95% of the time they go
there.  They can fit all four feet in there, even two of them comfortably
fit in at the same time.  They sometimes lean over one edge to look
around while they poop...so, that's what worked for me.
 
Good luck,
Angela (Minky and Pandora....litter box, we don't need
no stinkin' litter box...make mine a clean one)
[Posted in FML issue 3920]