What we found works for us are two types:
1) stackable baskets (as they have a low entrance, easy for them to get
   in and out of) and they are nice and wide (so they have more room to
   make themselves comfortable);
 
2) plastic bins with openings cut out of the two sides away from the
   corners (an opening in the center of the side, which makes it easy
   access for them).  We put newspaper in them, and clean them about twice
   a day.  At the beginning they used to go just outside but we wrapped a
   bath towel around the poopie basket and sprayed it with linatone-- and
   they got the message!
 
Dooks,
Tina
[Posted in FML issue 2694]