Hans,
I made a box out of thins steel sheet and filled it sequentially with lead. I welded a piece of tubing over a drilled hole for the bolt and then added sides that fit within the fuselage structure. You can make the vertical height as tall as you think you may need. A thought on the lead: Another approach rather then melting and pouring would be to use some
fine buckshot from the hunting supply store and after finding how much you need, pour some epoxy resin in to saturate it and bind it to the walls of the box.
Diagram