I cut a piece of MDF large enough to close the open face of the mould, and then fabricated a pair of aluminium rails 1/8" thick and 3/8" wide to form the shoulders of the raised portion of the counterwieights. These rails form a tight fit with the MDF blocks of the mould, and so whould prevent leakage of liquid lead befor it freezes. I attached the rails to the MDF board with screws, and then used Bondo to cover the screws and close any gaps under the rails.
