Cenbeth - great write up with picys, the process can be daunting when casting - I have tried 3 metods to produce a keel bulb - thought i would also share my experiences .
(1.Casting 2.Shaping 3.Filling)
1. Shape a wooden keel bulb and from it produced a concrete 1/2 mould (*leave concrete to cure/dry) then cast as two lead halves.
2.Cut lead sheet profiles to diminishing templates, glue together and shape to bulb profile.
3.Used 1/2 wooden keel mould to Vac form two plastic keel bulbs which can be filled with lead shot/or concrete - then stuck together.