I do this stuff out of my head as I go, I have no templates. Its really simple to get the size and shape of the plenum if you have the stock air box. Copy the the air box for where the flanges have to be and the size, the angle ralationship between the throttle bodies and the top cover that fills the hole in the hood, then make the top piece just a little lower so it clears the hood and doesn't rub.
Right now I'm working nites and during the day when I'm not sleeping I'm tuning up my Grandbabies '73 Kitty Kat. I just shoe horned a 6 1/2hp 4 stroke into it and tappered the tunnel, now I'm working on some traction and skis. When I'm done, I'll put together a list of parts, pics and some explaination of how I did it.