This may be hard to follow -- refer to view (C) of schematic 2:

You may be able to do the whole thing in one swell foop (fell swoop?) Imagine the same [1 1/2] X [1 1/2] X [something long] piece of hardwood. Imagine you're table saw's blade coming down vertically past the outer edge of the coaming lip in (C). This would hollow out the left part of the channel. Then set your saw up to come in horizontally from the left to hollow out the upper part of the channel. You will end up with the basic shape plus a rectangle of wood in the opposite corner that you can't get with the table saw. Now, just like cutting off slices of salami, cut off 1" pieces, varying the angle to form the oval cockpit opening. After each secion is cut off, finish trimming out that extra wood in the lower right corner on your bandsaw or scroll saw.

Hope that was intelligible. Dean