I started with foamcore and cardboard mold walls,  Legos are SOOOOooooo much better for walling up the mold, but may require some no sulfur modeling clay around the bottom to prevent leaks.   When making two part molds we have been putting airsoft BBs in between the mold halves to make index "nubs"  this works well,  Great tutorial over all,  I cast quite a bit and even learned something.