Answer:
The best litter pan is usually the trangle kind the fits in the corner of the cage. Ferrets prefer to "do their business" in a corner by backing up to the corner, so wait to see which corner your fuzzbutt likes and put the litter pan there {this is also the quickest way to litter train them}. Try to use a litter that is recomended for ferrets (my personal choice is Yesterday's News as it is made from recycled newspaper) as some kinds of litter can cause health issues or (in the case of the sandy kind of litter) your fuzzy might like to play in it. Don't be surprised if he thinks the pan is a new bed, this is normal, I have 6 males and they all have gone that route. It might be a good idea to have more than one pan especially if the cage has more than one level.
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Most ferrets I have met prefer the large square ones. The corner ones are too small for them to fit their whole bodies into