Whilst waiting for the epoxy to set on the inverted hull I turned my thoughts to how to cast the slug. I'm going to try the lost polystyrene method, with a plaster mould. Pouring the hot metal destroys the polystyrene former, allowing the metal to take it's shape. The advantage of this is that the plaster mould can be in one piece, but is destroyed as you get the slug out. This is fine, since I'm only going to make one slug, assuming everything works correctly.
I got some half-inch polystyrene (4.99, Hobbycraft), cut it (standard hacksaw) and sanded it (80 grit) to fit the CB cutout.
Next I got a suitable container (old seed tray, lined with cling film), and some plaster of Paris (3.99, Hobbycraft). I learned that it is important to mix the plaster of Paris in small amounts. It kicks really quickly and I wasted about 0.75 kg at the first attempt. I put some in the tray to form one side of the mold, inserted the foam and a sprue, and coated the other side. In theory (according to me) the plaster does not need to be all that thick. I'm going to stand it in common or garden sand to support it when I come to do the casting - the sand should hold the sides in.
Having read tales of what happens if hot lead hits water, casting will be after everything has had a week to dry. Will keep you posted. Meanwhile I'm waiting for it all to dry then I can scrape the plaster off the floor.
Sorry if this read like an advert Hobbycraft.