Just an idea, there was a man, a Vicar I believe, who built a big model of the SS Great Britain. The build was published in Model Boats as a series in the early 80s. I seem to recall he used old Motorcycle chains as ballast. Cleaned of course, they could be formed in to the shape of the hull and removed at the pondside, so making transport easier.Ordinary iron chain would do the same job and not involve hot lead! I use old Wheel weights from our local Tyre Stockist. I give them 10 Euros for the Coffee Fund and I get half a bucket full, Melt them to remove the metal tags, and scim off the scum and you have shiny"Lead" !! Another plus, it melts at a lower temperature than lead.