If you're concerned about spillage, I used sand inside 4 inch diameter PCV tubing cut to fit my old S10 pickup when I needed ballast for it. One could certainly do the same thing for an LS, but you need to make sure to have a positive way to locate it inside the trunk. For the S10, I just used nylon strapping wrapped around the tubes and ratcheted down to the rear bumper under the tailgate edge to locate the tubes on the S10. You don't want 60 pound tubes rolling around in the trunk, but I don't know how you'd secure them in the trunk without perhaps welding some tiedown points in the trunk.
Now that I have a 4x4 I no longer need to worry about ass ballast. The LS just stays home on icy days now.