Where do I find a big enough box or get my ferrets to stop using the entire base platform of the cage as their litter box?

My ferrets are litter box train. I also clean out their litter box twice to three times a day. They seek out the litter box to go to the bathroom but there are times where their box gets full, despite me constantly cleaning it out, and there are other times one of the three chooses to poop somewhere else... they... show more My ferrets are litter box train. I also clean out their litter box twice to three times a day. They seek out the litter box to go to the bathroom but there are times where their box gets full, despite me constantly cleaning it out, and there are other times one of the three chooses to poop somewhere else... they have decided that the entire bottom platform is now their litter box. Which would be fine if their poop wasn’t sleeping out of the bars of the cage and dripping to the floor! How do I get them to only go in their litter box or where do I find a box big enough to cover the entire platform and keep their excrement contained?
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