I was trying different things & was disappointed because Bella didn't like the tray for puppy pads. This worked for Bella:
She is really great about using the puppy pads, but occassionally she would miss on the edges.
So, I found a pack of kid's play mats. They are shaped on the edges like a puzzle, if you know the type I'm talking about. There were 4 or 5 in a pack & one fits exactly to the puppy pad with a little room on all four edges. It lifts the pad off the floor a little & really seemed to work wonderfully. They were really inexpensive. Hope it works for your baby.