I made an outie dent in the Dart carrying replacement torsion bars for the Challenger in its trunk. After that I took some high density packing foam sheets (about 3/4" thick and very dense) and stuck them inside the trunk, sliding them down the gap on each side.
I also had a rubber trunk mat in the Dart. I used the double-sided rubbery tape they make for holding carpet runners in place on tile floors on the bottom of it; that did a good job of preventing it from shifting. Then I took a couple of milk crates (real ones, not the cheap toy replicas they sell at wally world for storage) with rubber tape on the bottoms. They tended to stay in place except under the most severe braking or turning, with a little weight in them.