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.