I started mine off in a large Rubbermaid container, but moved them into an old pack and play this past weekend. They love it and now I can use the Rubbermaid for the babies I just couldn't help buying at the feed store last week ..... chicken math strikes again ..... plus 6 Australorps coming mail order from Meyers next week .... must stop ..... must stop ....
Check flea markets, thrift stores and maybe craigslist -- since you're not concerned about any of the attachments (diaper station, mobile, etc.), you should be able to find one pretty cheap. I did cut a small piece of plywood to put the food and water on, although the bottom seemed plenty solid. I've got an old window screen over the top, chicken wire would work too. The older chicks are 5 weeks today and wow can they fly!
Our pup was licking at the mesh side, so I reinforced that side only, the others are solid. The only funny thing so far is that it's blue fabric with small white polka dots, and the chicks think they're bugs there to play. We only have dogs so I have zero experience, but with the mesh side covered and a top over it, maybe the chicks would be safe from the cat?
Good luck, have fun, chicks are so much fun!