I read your note on the FML and I had the same problem.  At first I bought
a litter pan that was triangular, but soon I found out that didn't work.
Then I bought a rectangular pan, but that didn't work either.  In a last
desperate attempt I figured since I had spent the money on both litter pans
I would use both of them!  What I did was put the rectangular pan in first
and next put the triangular pan in with the lowest of the 3 sides facing
the rectangular litter pan.  And it worked she has gone in her litter for
going on 3 weeks!  Now I don't know if this will work for you, but it shure
worked for me.
[Posted in FML issue 1667]