I was at wal-mart the other day, and happened to find a perfect hide box. It's a small bucket made by sterelite. They only cost $1.97.
They are fairly easy to cut with a scissors, and you can make them any height you want. After you get it cut, take a lighter over all the edges to smooth them out a little.
It's not the prettiest, but they are cheap, readily available, and easy to clean. They are a good size for balls up to about 700-800 grams.
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