which you can have in whatever thickness you desire. I think McMaster carries it up to 2 inches. Using a bandsaw (ideally, though other saws will work) to start with, whittle on it till you are close to the shape you want, then a bit of grinding work. You don't need a perfect fit - you are going to bed that in thickened epoxy. Glue in place when it looks pretty enough.

I would use the flanged Marelon seacocks. You can get pretty long thru-hulls to match. Using just a mushroom fitting and a nut, then screwing on a Marelon ball valve would only work for me if they were very well protected by something solid. Its easy enough to set one up on a piece of plywood as a test and then step on it or kick it hard - they snap off fairly easily. Not an ABYC or CG approved installation. The flanged ones on the other hand are pretty robust, and meet both CG and ABYC standards - I think 500 lbs load at the extreme dimension worst direction for 30 minutes.