A couple of years ago we added abut 250 pounds using recycled lead shot. The shot is clean and you don't have to melt it to get it to conform to a certain space. The 25-lb. bags are about 4.5x8x2.25 inches ? or about .3 pounds per cu. in. This may help you plan the container(s). Our frame rails were constructed with 1/4-inch steel and segmented, so we were able to put some lead in them (not pictured herewith). Our 18-gallon water tank is made of 1/8-in. steel with a 1/4-in. bottom, and it contributes, too.
The square tube pictured here is made of 1/2-inch steel. I bought it at a steel shop as a remnant. We welded 1/2-inch bolts from the inside and they go thru to a backer underneath.
We found the shot at Redlands Shooting Park, 2125 Orange St., Redlands. They're open daily and it's a well-run operation. Phone 909-335-8844. They move a lot of shot and are sometimes out of it, so I'd call first before you venture there. They charge $1 per pound. Anyone local can have the free bag illustrated.
(I can't post the pictures ? and they are size right.)
