12mm Marine grade plywood is $60.65, Premium is $29.75 or $35.25. I don't know why the bigger piece of premium ply is cheaper. I think I'll just pick a piece that looks nice.

Many builds don't have two layers underneath the centre section of the board, rather they use offcuts to form the rim and glue it together. I think I might make mine out of two circles of thinner (10mm? 8mm?) ply glued together. This gives me continuous grain in the gutter, and is faster to build.