I bought a six pack of Sterilite #1805 large flip top plastic boxes for about $22 from Amazon. I think they only come in six packs.
They are Length: 13.12. Width: 7.62. Height: 4.5. You guys from the great white north will have to convert to CM.
They are a tight sqeeze fitting in the underseat box door but fit well once inside. As a matter of fact, if you put one in and turn it sideways, you can put in a second one and then turn it back down. Pretty much splash proof and wont wear on the floor.