I've been adding weight to things for a long time. Be it a cheap plastic knob on a tool, to the lathe I built back in 2002. I fill a cavity with #7 1/2 (or finer) lead shot to within about 1/8" of the top. After leveling the shot, I pour thinned expoy into one end of the cavity, letting it flow down and across, until the cavity is full. This forces out any trapped air and renders the object solid and heavy. If the thought of lead bothers you, you can substitute steel shot. I've filled hollow cast aluminum and plastic parts. I made lathe rails from aluminum angle and filled the channel with the shot/epoxy matrix. The base under the lathe headstock was filled with the matrix to help quell vibration. The tailstock was a hollow aluminum casting weighing one pound. It now weighs 10 pounds. The mantle hook shelf could be routed out on the underside and filled with the matrix to add weight. Maybe a bit involved, but effective.
I've gone out to find myself. If I return before I get back, have me wait for me.
"Sawdust is Man Glitter"