I just got suckered into adopting yet another homeless ferret (doesn't take
much) and he came with a great cage.  His owner bought a couple of rabbit
cages 2' X 3' X 18" high (I think) that cost maybe $18-25 at the local feed
supply store.  He stacked one on top of the other, wired them together, cut
a hole between them, added a ramp, and had a roomy, two-level cage (more
platforms, etc.  can be added according to your imagination).  One added
advantage of this cage design is that there is no way for a ferret to fall
very far inside the cage (18" max), since the top level has a complete floor
with only a hole cut through.  I recommend floor tiles (the ones you can buy
in 1-foot squares and glue down yourself) for covering the wire mesh floor
on both levels -- very easy to keep clean, and stylish too!
 
BTW, I am noticing an interesting characteristic of ferret math: it seems to
be one-way.  That is, you can keep adding ferrets as long as you like, but
subtracting them is impossible.  Once you've lived with (n) ferrets, (n-1)
or anything less seems too few.
 
-- Kim, studying for a degree in mustelid mathematics
[Posted in FML issue 1955]