A really CHEAP solution I found at Wal Mart.

Not pretty, but functional. It's an underbed storage box.

I've ended up using it minus the lid, which is too large for the space available to be practical. I just wanted something to keep stuff from rolling under the seats, without spending a lot of money. Works for me. Fits neatly, it's 66 quart/62 liter. 39.5x20.25x7". Probably too ugly for the taste of many folk though.