Whatever you put in the back, make sure it is secured so that it doesn't get a chance to fly around in the event of an accident.
When i was a kid, the favourite trick was to put old grader blades in the back of the pickup... the local municipality had a pile of them sitting there and they were NICE and heavy... lay about 4 of 'em in your box and you were ready to go!
Until Keith drove through that corner and stopped hard against the ditch by the fence... 3 grader blades came right through the box, cab and seat of his truck, narrowly missed him, impaled his dash and one even got through the firewall into the engine compartment.
If he had been carrying passengers, it would have been more than an abject lesson for us all.
I had a big plate of steel that my welder son-in-law modified to slide into (and lock into) my fifth-wheel hitch rails. His weight is a 14" I-beam that stretches across the box of his truck, bolted through to the frame rails so it won't move.
Hopefully, none of you will ever have to worry about what happens to your ballast in an accident, but thinking about it now might save you from regretting it later.