I've been adding weight to things for over a decade by filling voids with #7 shot and pour able epoxy. I started this method to make hollow plastic knobs that tool manufacturers put on tools these days more durable. My shop built lathe has many of the parts filled with this mixture and weighs in at 120 pounds, even though a lot of the pieces started out as plastic or aluminum castings. The lower rails are aluminum channels filled with the mix. I fill a void to within 1/8" with the shot and then pour in the epoxy from one end or corner. The slow setting epoxy flows down and across the void expelling air as it goes. Once cured, the piece has nearly the weight of steel or cast iron and a lot of vibration resistance.
I've gone out to find myself. If I return before I get back, have me wait for me.
"Sawdust is Man Glitter"