I have this too that I use for the Spring/Summer/Fall months, as the fake grass needs to be hosed down 1 or 2 times per week and in the winter I can't do that here in NY
DoggiePotty Indoor dog bathroom. Doggie potty a training solution.
In the winter, I have a large piece of lanolium (on top of the hard wood floors) with the liter boxes and pee pads down.