I have a piece of plastic carpet runner that I place behind the door.  I
keep the edge curled up against the door to prevent any puddles from
running off into the cracks between the plastic and the wall.  The
advantage is, when I open the door, this pushes the plastic, poop and
all, away from the door instead of spreading it.
 
Not a perfect solution, but it protects my carpet.
 
Amy
[Posted in FML issue 3684]