I know some are not big on a cardboard box as a brooder, but ours is working well. We keep a nice layer of pine shavings down and have had no trouble with a wet bottom.
I wanted to let others know where we got it.... for free I might add !
At a furniture store! Our local furniture store lets us have boxes for free....recliner boxes - like a Lazy Boy are nice--they are usually square and have very tall sides which work well for a brooder box
So, if you can't find a box large enough, ask your furniture store if they have boxes going in the dumpster and you could be in business ... for free!