I believe "Tablecraft" use to and maybe still makes a 5"-7" deep bus tub that has flat bottom and smooth sides. We use to use them for AFS and mice. Till our local suppier stopped carrying them.. Now we just use the a converted tote for them and I think there is even some Kitty litter pan racks out there too.. All of those are good options..

We love the mortar tubs for rats though..make sure you get the ones with out the hole in the end. They put it there to let you hang the tub up, but it makes for a great place for rats to ruin a tub.