The hole should be large enough for the ferret to move in and out of easily. This type of box helps to keep litter from being scattered all about as they like to dig each time it is changed. These boxes are easy to find at most Target or K-mart store and are not very expensive. If possible, the dust-free kind is best. Another very good choice is the recycled newspaper type that comes in pellet form.
Ferrets are very finicky about where they do their 'business'.
Be sure to clean the litter box every day or at least remove any large amounts of feces. Litter should be replaced entirely at a minimum of every 3 days. The litter box should be washed and disinfected with a solution of water and bleach at least once a month to avoid infections or contamination. It's very important that your ferrets have fresh clean water at all times. We have tried many methods and products and have found that the water bottles made to hang from the outside of the cage work best.