I grew up with my father raising Ferrets. It is true they can fit through a hole the size of a quarter so always keep that in mind. They also go to the bathroom in corners. So place a litter box in the dog's carrier in the corner if possible.
I recently gave a ferret to my father for Christmas named Measly Weasal. She loved her hammock. So I would modify the cage to put a hammock in it. I also got her a leash and harness to walk her.
A rabbit cage is large enough as well to keep the ferret in if you allow for enough exercise outside of it. I kept Measly in a room to run around in.
The suggestion of chicken wire is a good one. Hope you have fun with your Ferret. I miss my Measly but my dogs tried to eat her!