We found this marvelous thingie to replace our ferret cage. http://www.dogkennels.com/dog-kennels/dog-exercise-pens/softsideplaypennavytan.cfm

It weighs incredibly little, pops open, folds in a jif, and when all zippers are zipped, even a ferret would not easily get out, unless it ate its way through. Cats won’t. Dogs—well, generally won’t. And for those of you who like to take a trip with your pets of almost any sort, this is big enough to contain the food dishes, litter pans, water, you name it, plus a sleeping mat. Our big multi-shelf thingie is good, it rolls, but it’ll take two or three fingers as a pinched offering before it stays put. This one is a grab-it-shake-it, zip-it, assembly and easily fits under your suitcases in the car trunk.