The best way to get the maximum amount of lead in the space is to make a plaster mould- fill the space with plaster of paris, having made sure you use a relaease agent- I have used Vaseline -and then make a female mould from this. This should be absolutely bone dry -put in in a low oven to ensure this- before pouring the moulten lead. This will give you a lump of lead exactly the same shape as your keel space. You can plane lead with a sharp smoothing plane using paraffin as a lubricant to adjust the weight.
If the shape is suitable I have used lead flashing laminated with epoxy, which obviates the need for any moulten lead. Vic Smeed did a good description of the moulding process in a little booklet published in the seventies about small model yachts. I can't lay my hands on my copy at the moment but will search it out if you were interested in copies of the relevant pages. There must be other descriptions in books about model yachting.