I've seen heavy duty produce boxes that were in the 12-18" range and had a waxy coating--those would be perfect. But darned if I can find them now that I'm looking.....
For those of you who are looking for milk crates, you can post a wanted ad on Craigslist or post on your local Freecycle and I'm sure that someone is giving them away. I posted an ad on my local freecycle last week and within an hour, someone emailed me and has supplied me with an unlimited number of milk crates.
used a 1x12x10 pine board
1x4x10 pine board
4 cement blocks
attached 1x4 to 1x12 sat that on the blocks placed the milk crates on their sides
just empty to clean fill with new straw and replace