I've made some pictures with tape measure if anyone is interested in making a sailing deck like this one. The fit of mine is actually pretty good and although I was originally a little concerned with scuffing where the deck meets the ribs, I haven't seen any evidence.
As seen in the earlier pictures, my deck begins at rib #3 and ends a little before rib #6. If I'd had a longer piece of wood I probably would have made the deck reach back to rib #6... but I didn't. As it turns out, this seems to be about the perfect length and I can place my electric pump on the boats deck forward of rib #6 instead of back between #6 & #7 where I like to store gear. So the 'shorter' length was a little fortuitous for my purposes. Beginning the deck at rib #3 allows the rudder peddles to be mounted in the normal fashion just ahead of rib #3. This way you can steer with the peddles from a seated position amidships, or from the tiller with extension from anywhere else in the boat. I've often used the steering peddles by hand when up forward making adjustments to sail rigging while underway.
When I made these pictures with measurements I made them in reverse of what I had intended. So instead of looking at the board from rear to forward, the angle of view is forward to rear. The board tapers out gradually from rib #3 to it's widest point at rib #5 where it tapers back in pretty quickly.
If you make a rough cut rectangle on your board a little bigger than these dimensions you will be able to fit it in place resting on the edges of the ribs. This will give you the cut-out lines for the ribs. Next cut the rib slots so it lays nicely on top of the ribs. Once you have it resting on the ribs you can decide on the taper you want it to have. I think mine follows the outer stringers pretty closely. The stringers ride higher than the boats deck so you will need to taper the edges of the foam (or whatever you use) for a nice fit.

Sailing deck, looking forward to rear

Rib #3 lip

Rib #4

Rib #5

End of board between ribs #5 & #6
Here's a link where you can see all these images larger and some additional lengthwise measurements:
http://www.dennisleephotography.com/Boa ... &k=dsFPL9T
Dennis