The backing block is made from solid mahogany. I drilled out the block with oversized holes. Next, I coated the block with epoxy and refilled the holes. I then wrapped the block in fiberglass.

The bolts are counter-sunk into the block so it will lay flat. Although the pic shows stainless bolts, I plan to use bronze (didn't have any on hand at the time).

This shows how the seacock will mount on the block.

The good news is that I was able to mount the seacock all by myself. This is kind of an interesting project for one person to attempt. The bad news is that I already have a hole going thru a backing block. I couldn't use the new backing block without drilling another hole in the boat and closing up the old hole. Instead, I took a 1" drill bit and opening up the 1/2" hole. Then, I epoxied inside the hole to seal it. The next day, I redrilled thru the epoxy and bedded the seacock with BoatLife. I considered using 3M 5200 but figured that there is a 1 in a million change that I will want to remove the seacock and replace it in about 50 years. Since I didn't use my new backing block, I wasn't able to countersink the bronze bolts that would hold the flange in place. So, the new seacock is in place, sans bronze bolts.

Notice in the picture above that I sanded the paint around the hull... the grey color is from the barrier coat, Interlux 2000/2001.