Dog Housetraining Supplies: Protection Tray for use with Piddle PadsŪ at Drs. Foster and Smith that's the one I got for my Truffle (in playpen) and whole pad will fit perfect!
living room and kitchen has smaller size than that (I picked at Petco for $19 came with some pee pads ) these are little smaller size so I make sure to fold good so no edges comes out from tray for Truffle to pick and distroy the pad.