While I'm not too familiar with Coosa board, I would recommend that you use marine plywood instead. I believe Coosa board has a foam core and is not going to have the compressive strength or tensile strength that marine plywood is going to have. I also believe that marine plywood would be less expensive than Coosa board.
I would make the replacement outboard mounting board out of two pieces of 1/2" plywood, since that should have more than enough strength to support an 8 HP outboard.
I would make it by laminating the two pieces together with epoxy, and then screwing them together. After putting the screws in, I would glass over the whole thing using kevlar* cloth, to give it abrasion resistance and to encapsulate the wood and prevent water from causing any problems with it in the future.
When you drill the mounting holes, I would drill them oversized and fill them back in with thickened epoxy, to prevent the water from intruding through the fastener holes. You should paint or varnish the board to protect the epoxy from UV-degradation.
* If you're going to sand the board to finish it off, use a final outer cover of fiberglass, as kevlar is a bastard to sand—basically, it doesn't, since it is so abrasion resistant, it shreds and leaves little spikes sticking up all over instead.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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Still—DON'T READ THAT POST AGAIN.