I have also used puppy training pads in my cages and have found them quite
useful.  I might suggest you find some that don't have such a strong odor.
Ferrets probably don't need that smell to attract them to their favorite
corner.
 
As for being sealed on four sides to prevent leakage, that probably refers
to puppies urine leaking from soaked pads.  (Puppies have much larger
bladders than our little varmits.) I was worried about the stuffing getting
all over the place, so I "re-sealed" my pads with duck tape, or you could
just tape them to the floor of your cage and that would serve the other
purpose of keeping them in place.  I tape the pads along my walls to
protect my carpet.  I use masking tape there since it is more carpet
friendly.
 
Hope this helps!
Britta
[Posted in FML issue 2471]