Walmart offers the Iris Travel Cat Litter Box for $11.69. Opt for in-store pickup to dodge the $5.99 shipping charge. (Amazon offers it for the same price.) That's $3 off and the lowest price we could find. It's foldable and features a zipper top.

Update: It's now out of stock at Walmart; however, Amazon still offers it for the same price with free shipping for Prime members.