Speaking of water solutions, I bought one of the slant backed 50L water tanks from Iron Man. In our troop carriers it fits perfectly behind the front seats. I have internal cabinetry that holds it against the seats. If you had nothing back there, or the space behind the seats was filled, you'd have to work on a solution. It would be easy to bend up some straps. It's a great temporary solution which means it's now permanent.
www.ironman4x4.com

50L Tapered Water Tank - 13 Gal
Water in the backcountry is liquid gold, so set up your vehicle with an IRONMAN 4X4 Water Tank. The IRONMAN 4X4 50L Tapered Water Tank is built tough from food grade, BPA free, UV Stabilized Polyethylene ensures you can enjoy clean, taste free water. This is a perfect size tank for that void...
