Wayne thank you for the in depth reply and added information that you're giving me.
I think I've decided on 18mm birch ply with the extra pieces (mounting the drivers flush, bracing the cabinet and making the port) made from 15mm birch ply.
I'll probably round over or chamfer the edges and then finish with teak oil or wax polish.
I've ordered the B&C 12PLB100 drivers so need the crossover components and binding posts.