The last part of the puzzle will be a surprise to most readers, but what do you expect from a dew infecting GBH on a lathe? The spindle is wrapped in plastic to keep it clean while I shoveled in concrete until just below the bearing seals. There are three factors that control vibration in physical systems, which means that it is shake and / or poor surface finish when applied to a lathe. These are mass, stiffness and damping. I participated in the stiffness by putting on top and back covers. Concrete adds both mass and damping to the unit.
Lead would be the best material but perhaps excessive. Molten conduit of the required volume can gently heat the seals beyond reasonable limits and / or cause the iron casting to crack due to sudden and uneven heating. Preheating the head can help avoid cracking but would impair the seal heat problem.
At a more practical level, epoxy granite should be a good choice for the filling instead of concrete. Epoxy granite is a mixture of granite gravel (or similar material) bound in an epoxy matrix. It becomes a popular material for the construction of single machine tools instead of cast iron. Cast iron is cheap in quantity but expensive for an offs. A steel frame can be molded inside an epoxy-granite machine tool frame for increased rigidity.
I leave the cylinder head until the concrete has set and dried thoroughly. If it was covered too early, condensation may occur on the spindle. Drying can take up to two weeks or so. When it has dried out, I pour a thin layer of old paint over the concrete to seal it and prevent dust.