You could try putting something in the middle of pad for them to lift their legs on... A sort of target for them to aim at.
I would think a plastic soda bottle or milk jug would work well (might want to put some sand or gravel or something inside to weigh it down a bit).
I've seen mini fire hydrants at our pet store... I think it's supposed to be a treat jar, but that would work too. I would be just like outside!!
