Hi,
 
For a collapsible cage, try Martin's Cages,
http://www.martinscages.com/products/cages/ferret/.  Scroll down to the
bottom of the page.  have the large one that is about 30 x 18 x 18, but
they also have a smaller one as well.  I've been very happy with it.  It
even has a ramp.  It works great as a sick cage too for when you need to
separate them, but they can't or shouldn't climb ramps.
 
It was large enough to fit two full grown cats (minus the ramp) when we
moved to Ohio last month.  Fozzie rode in his carrier.  The ramp clamps
on with rubber tipped clamps.  They come off easily, so if you get the
cage, glue them on.  The last thing you need is an intestinal blockage
in one of your ferrets.
 
They also have some sort of playpen at the very bottom of the page.
 
Someone mentioned outfitting a carrier with the corner litter boxes the
other day.  It involved drilling holes and stuff.
 
Too much trouble, IMO.  I have had great success with a medium/large pet
carrier and the Van Ness small cat litter boxes.  I took two litter boxes
and outfitted the bottom of one with four pieces of sticky velcro.  I
left the velcro together (stuck to the box) and peeled off the other
backing.  I simply pressed it into place at the back of the carrier.
Walah!  The one box stayed put for several years, and the other I placed
inside it, so I could pull it in and out for easy cleaning.  I also hang
a hammock, but high enough so they still have head clearance to use the
box.  This has worked great for me for 7 years , through numerous vet
visits, and even ferret shows way back when.  Three to four ferrets
shared this space comfortably, but for no longer than 8 hours.
 
This is a nice alternative to a travel cage.  Plus it offers more privacy
for the ferret.  No poking and prodding fingers, and less chance of a nip
or germ transmission.
 
I hope this info is helpful!
 
Dodie (formerly in NJ, now in Ohio!)
[Posted in FML issue 4134]