I don't like to use wood backing plates for below waterline
seacocks because of this very reason: Wood will eventually rot if kept wet and unprotected. A rotting base plate in a critical application can lead to a leak.
I would use a 3/8 to 1/2" thick fiberglass/epoxy plate instead. They are easy to make even with a curved contour and will be impervious to water, expand an contract similarly as the hull and strong enough to be through drilled and bolted.
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