I have used various things over the years to include concrete parking bumpers, 5 gallon buckets filled with cement , 4in thick x 12in diameter steel plates with handles welded on them, firewood, old engine blocks, scrap tractor trailer rims. I always used ratchet straps on anything I had in there. Once my ex had an accident with the 5 gal cement filled buckets in the bed. The straps broke sending buckets into the front of the bed, bending it but the straps took enough energy out of them to keep them from coming thru the cab.
NOW I ONLY USE SNOW.
Like other said it melts by itself and you don,t have to worry about coming thru the cab and killing you. I go to the closest shopping center were they use front end
Loaders to remove snow, bring the guy a coffee and have him dump a bucket of snow in the bed. No back ache, no projectiles to worry about.