I've been struggling to get something adequate in my ballast box. Cement really wasn't heavy enough, and I wanted to still be able to carry a chain and other stuff in the box also. I didn't like the extra bulk of the box extension either.
Today, I had 10 pc of 1 inch steel plate plasma cut to fit in the ballast box (22x16). At almost exactly 100 pounds per plate, I got 1000 lbs using only 10 inches of the box I like that all of the weight is below the axle
Here's the best part.... less than 30 cents per pound cut.
It's good to know people in the metal fab world