Personally, I would dispense with the plywood and build up extra thickness with fiberglass and
epoxy or use one of the high density
epoxy composite sheet materials as a backer because plywood is more likely to fail in this mode. That said, using plywood backing blocks is pretty tradional. You need to cut the plywood to shape, and over bore the holes in the plywood for the thru-hull and bolts passing through the plywood. Then coat the plywood on all sides and in the bolt holes with several coats of a quality epoxy like WEST or MAS. Then fill the holes with thickened epoxy. I would laminate a piece of cloth on the surface of the block that is against the feet of the seacock to keep that surface intact if the bolts are over tightened. Then I would rebore the holes for the bolts and thruhull. I would either epoxy the block in place or intall it with 3M 5200 either of which makes a pretty permanent installation. With the block in place, bore through the hull for the bolts and thruhull and drill the countersinks for the bolts (and thru hull if flush mounted). I would use a Quetip and put a little epoxy on the exposed edges of the glass exposed by the drilling. Mask around the holes to help with clean up and then simply goop the thruhull up and install. The actual caulking is a little controverial as 3m 5200 is a very good product but it''s permanent in nature qualities are not exactly what you want if you ever have to remove the thruhull.
Jeff
