Hey Drone, When SWMBO isn't riding monkey, I use two 40 lb plastic sacks of water softening salt. I'm not doing the rugged outback, but if I was I'd move the salt into a more secure, flexible container. An old Navy duffel bag or air force flight bag comes to mind. I've hit some pretty big potholes and the salt and/or my wife haven't bounced badly at all. The salt will "flow" into any shape, so long as you keep it dry, of the container you put it into. It is certainly cheaper, and safer than melting lead. It should be pretty easy to lash down inside the car too.
"Seika" - 2013 DL650 + DMC M72D Sidecar
"Gracie" - 1983 R100RT 60th Anniversary Edition
"Dusty" 2011 Adventurized 4Runner
"Adventure is worthwhile." ~ Aristotle Airhead #14449