Pet stores have potty trays that come up around the sides so nothing will get on the floor. I use a flat tray with Sass and under that I have a clear piece of shower curtain liner that comes out about about 4 inches from her tray so that if she misses it gets on the plastic shower curtain and not the floor.
You might be able to put down a piece of a shower liner under her pee-pad and see how that works.
Good Luck!
