If you want the weight to be permanent, have someone "sister" some extra metal to the box, making sure you distribute the additional weight evenly over the box. In other words, weld the metal on each end, and maybe some on the rear, but not some on one end and some on the back.
A couple of times when I wanted a little more weight, I tied a heavy piece of scrap steel across the top of my box where the rippers usually reside. I took those out. It worked pretty well, but would be better if it were in those other locations.
John