When I had my pickup (I had pickups for 24 years) I built a frame out of 2X4s to the dimensions that had one fore of the wheel wells inside the bed, one aft of the wells and two longitudinally snug inside the insides of the wells and wedged between the front of the bed and the tailgate. I secured it and it could not (because of the fit) move forward, aft, or sideways. It had a board floor and was dimensioned to accept 2X8X16 concrete patio blocks. I then laid two layers of the patio blocks and put a “lid” on it with decking screws. It took some work to design and build, but it worked great for years on end.
That ensured that the “system” was well secured in the event of an accident (none of the blocks or the parts could move within the bed), and gave me a slightly higher floor so I could use the bed normally. In addition, I could put lots of weight in, adjust the weight, and at the end of the season I could simply unscrew the lid and remove the concrete blocks one-by-one then remove the framework.
I also carry Gatorade bottles full of dry sand. Sand that can become wet exacerbate rotting greatly. The Gatorade bottles prevent this, as well as being great for spreading the sand when needed.
Whatever you do, be sure the weight you put in is well secured. Things like concrete blocks can become deadly projectiles in an accident.