Michael,
 
Theferretstore.com sells a hammock called "TheFerretStore.com Ultimate
Fleece Hammock" which advertises that it's big enough for 8 ferrets.  My
4 sleep in a regular Marshall's fleece hammock, and I've seen 2-3 in the
sleep sack (which doubles as a hammock, they can choose to be inside or
on top of it).  Our Marshall's fleece hammock is bigger than the cheapest
super pet plain hammock, but it's not huge or anything...remember, all
the ferrets crunch up together when they sleep and don't take up so much
space!
 
Happy Hammock Hunting,
Chana
[Posted in FML issue 4095]