Cheap but humane housing?

Anyone have any good ideas for cheap and humane housing? Preferably, this would be stackable. I was considering plastic tubs with lids but I am concerned they would chew through them.

Rats?
I use plastic lid-latching tubs, and haven't had a problem yet. Keep zip ties and some 1/4 inch hardware cloth handy incase you see signs of gnawing. I use 64 qt tubs, and everyone gets a fleece hammock (just use a suitably sized scrap, you don't need a fancy purpose made one) hung from the hardware cloth that covers the opening I always cut in the top.

Rats NEED to chew, so some wood, or hard dog biscuits might prevent damage to the tub.
I feed our leftover pork rib and chicken bones, and they like chewing those. You might want to do your own research if you are concerned about safety.

My rats live outside in a roomy CritterNation in the summer, but that cage is just too big to deal with in our avail indoor space. I like the tubs because they stack, and interchangeable, and quick to clean (daily). I keep two females per tub. The males and females run together in the bigger Martin's cage when I want the girls re-bred.

Build a rack with concrete mixing tubs, 6 high and install a water system, it will make your life a lot easier. There are many DIY out there here is one of them http://arbreptiles.com/cages/rat_rack.html