If what you want is to store your lead in the most compact area possible, long angle iron molds are the ticket. They stack with very little air space.
I want to cast continuously preheating ingots on the rim of the pot and adding them as space permits without impacting melt temperature much. The Lyman bricks are perfect for that. Angle iron about the same length would be good too.
I will agree that you need to have a few of the smaller molds available to keep moving if you are smelting in 100+ pound pots. Of course, bigger molds take longer to cool, so I guess you need a few of them too.