I seem to recall 7 ply baltic birch plywood. 20 years ago when I was thinking about making a top for 2 sideways Holley's I was going to make a spacer out of wood or nylon with an aluminum top. Then I realized I would probably need thermocouples and O2s in all 8 cylinders to figure out the extreme stagger that manifold needs. That is why a new, unused manifold and both tops are still in a rubbermaid bin.