My hamster goes to the bathroom under her water bottle?

I have the Habitrail Ovo for dwarf hamsters. There is no bedding under the water dish because there's no need to put any there because its a very small area only for drinking. She poops and pees under it and it smells bad and I have to wipe it out every day multiple times. It's very gross. How can I make it so that she goes to the bathroom where there is bedding? Any ideas would be helpful!