The west booklet is a great place to start. You can do a search on this sight about transoms and you will find lots of conversations about replacing transoms and most of your questions would be answered. If I were you I would build up the outside wall to 3/8”thick then either 2 sheets of ¾” or 3 sheets of 1/2” either marine ply or composite materials. Now a days most of your new boats transoms are made out of composite Coosa composite
www.coosacomposites.com/ is the best and most expensive out there and would last probably for ever. Marine ply is cheaper and will last 20 years or more. If I were to do it all over again I would use Coosa not because I don’t like marine ply but because of the possible re sale of my boat 10 or 15 years in the future. The glass you need is woven roven like 1708 or 1808 and epoxy look as a few try
www.shopmaninc.com You should make the inside skin approx ¼” thick and build several extra layers all around the edger many of us added knees to also add support. Do you homework First there is plenty of info here to help kind of like carpentry measure twice and cut once. You can also view several projects on this sigh under Seacraft galleries that will show some transom re dues.
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