Hi,
You need to make a wooden plug to the profile you desire, then use it to make a mould out of plaster of paris. make two half section moulds.
Leave the moulds for a couple of days to make they are absolutely dry before using it. If there is any moisture left in it when you pour molten lead in, it will shatter with the risk of molten lead being sprayed around - and on you - it hurts!
Then, wearing protective clothing, face mask, gloves, boots and apron melt some lead, I have use an aluminium pan on a camping stove - outside to keep away from the fumes - poisonous - and pour into the mould VERY carefully.
Allow to cool and tip out. Drill through the lead for fixing bolts to clamp onto the keel, fill, file and emery to shape and paint!
Simples! - but potentially very dangerous, so be extra careful and think through each step.
Ian