For a thin, flat container, the Water Bricks look like they would work well. Haven't tried one yet, but they look pretty decent. For attachment, how about a ratchet strap across the rear of the back seat? I have everything ratchet strapped down in the back of my Jeep and have them hooked to the seatbelt mounting points, body edges (no plastic back there), seat mounts, anything I can hook to.