Stacey,
 
I invested in a grommet-setter and a bag of grommets -- less than $10 at the
hardware store, including about a year's worth of grommets.  Put a metal
grommet in each corner of the sack, cut an appropriate length (varies by
cage size) of flexible light-weight nylon rope for each corner, thread it
through the grommet and tie the ends securely.  Both the grommets and the
nylon rope survive the washer and dryer with no problems.
 
For hardware, I hang a metal shower-curtain hook (also cheap!) in each
corner of the cage; they're big enough that it's easy to slip the rope
loops on and off the hooks without having to climb into the cage.  I used
to use s-hooks, but since I'm not very coordinated I was forever knocking
them off the cage trying to install or remove the sleep sacks; for the
aim-challenged, shower curtain hooks (big pear-shaped things) are much less
frustrating.
 
What a good Friend of Ferrets you are!  Have fun with your project.
 
ellen & the Gang of Three
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[Posted in FML issue 2221]