I would suggest either the cat pan idea which I have used in the past but as a hay area.
Or
One of these two litter pans-
Super Pet Litter Pan Hi-Bac - Ferret.com Marshall Litter Pan for Ferrets Lock-On 14x14x9 - Ferret.com
The second one will take up one entire grid space of 14x14 so unless you have a really big cage, it may take up too much space.
Also I tend to avoid buying anything from stores like PetStupid or PetNotSoSmart. All you are doing when buying from them is supporting the sales of live animals. Ferret.com does not sell live animals and their products so far have been pretty good.