I have been replacing the old wooden sea cock backing plates with fiberglass backing plates. My kids and I made a mold and lathered up 20 layers + of fiberglass. Cut that with a table saw and made new backing plates. We put the good side facing up, creating a very smooth and flat surface to put the seacocks back on. The old wooden plates, although nice teak plywood, were showing signs of decay. A few were completely covered with caulk, a good sign that they leaked. It was impossible to get them off to rebed them without messing them up further. Here are some before/After pictures.
I didn't bother with nice circles, but cut octagons on the table saw. They are epoxied in place with thickened epoxy. I took a grinder to the areas first to remove old caulk and clean up the fiberglass before installing. The backing plates are almost 1/2 inch thick, about the same as the wood.