Have you looked at the storage containers that they sell at like WalMart and Target? You can get them really inexpensively there and they have sizes that are really low and long as well. I buy my litter boxes that way, they are durable too.

This is from Target, and is a set of 6 with lids, but I buy them individually and they're under $5.00 each.
http://www.target.com/gp/detail.html...sin=B000AVS7OK