There's more engineering involved in the construction of these things than you may first think. Take a look at where the boards mate - note that they are designed to shed water. In a 3-board set-up, the middle board is taller. This 'board' on my boat is actually 2 pieces of teak held together with glue and mechanically via 2 backing boards screwed on the cabin-side. This keeps the section from warping. Also note the endgrain of any section that requires you to use two or more boards (like to house the louvers for example). The end-grains are purposely opposed, again, to discourage warping.
I had a shop custome re-make mine and have long-since lost the contact information, but I do recall I had to give them very detailed milling instructions. They also used a pre-fab louvered section which kept their price down.