For a long time before I started to store stuff under rear seat of my jeep I used heavy duty waxed fruit card box reinforced with duct tape and home made dividers. It was great for everything I needed and in case I needed to start the fire in emergency situation it was a life saver. Also lined with garbage bag it can be used as water storage or wash basin. Some of my friends swear by milk crates and USPS mail boxes (yes to have one is illegal).

Matt