I discovered that a pair of Rubbermaid cargo bins fit nicely under the back seats. I think it's Rubbermaid model 3304-40. I bought mine years ago but I see that they're still available on ebay for $25 each with free shipping or Amazon for $33 each with free shipping. Looks like this:
If you don't put anything too tall in the bin, the bin can slide out from the end of the seat without lifting the seat up:
If you don't put anything too tall in the bin, the bin can slide out from the end of the seat without lifting the seat up: