James and I plumb-bobbed where the hot air duct passes through the coverplate, cut the hole and then fabricated a mould for the splitter from medium-density foam. When that was done we wax-filled the divets, applied multiple coats of PVA and carbon fibre with epoxy resin. It is likely that some more will be required before it's done, but it's a solid start to the shape.
Tomorrow we'll drill, prize and scrape out the foam; then trim, smooth and reinforce the part with more carbon if necessary.