I use car body repair filler to make jigheads. Isopon P38 stands up well to the temperatures.. but stay well away from the fumes when melting lead, outside is best.
to make a lead mould, get a couple of small containers just a bit bigger than the lead length and about as deep, then get an inline lead, put a toothpick through it, and coat liberally with cooking oil... Mix the filler and fill one container, then lay the lead/toothpick centrally across the top while it's still soft. Do the reverse for the other side and you then have a two part mould which just needs a bit of work to make somewhere for the swivel to go, then drill a pouring hole and a vent hole. You also need to make sure the two halves marry up... after the two sides are made, put a lead into it and close it up, then mark the mould so when you pour it all lines up.
When pouring I put the mould into a small vice otherwise you get a frilly lip around the whole lead.