Unfortunately even though the manufacturers intended the rear seat footwell to hold, well...feet—it often just ends up holding empty cheetos bags, or my personal favorite for a driving snack, corn nuts. It's about time you did something with that space, and why not store that 40L of heavy water that would normally end up on your roof rack, or mounted in a leveraged position on the rear of your vehicle where it should be—centered between the axles and down low. The Front Runner Footwell Water Tank ($275 MSRP) does just that, and it does it quite effectively. The water tank has a cutout for your transmission tunnel (if you have one) and features a top mounted filling receptacle, and an outlet on either side fitted with a quick-disconnect, but a garden hose adapter can be fitted as well.
We're constantly fighting over who gets to use the footwell tank we have at the office, it's great use of space and good construction make it a solid buy.