Yep, either will do. When I did the four under the sink I used a scraps of 3/4" fiberglass sheet for backing plates. It would be hard to go wrong with anything 3/8" to 3/4" that is either waterproof or can be epoxy coated to waterproof it. The hull is plenty thick and the backing plates are primarily used to provide a flat spot to seat the seacock. The epoxy with filler is just to fill the gap between the curve of the hull and the backing plate. I used WEST systems with their high density adhesive filler to make a paste.
I used new bronze everything and threaded the fiberglass backing plate to accept bronze 1/4-20 hex bolts for the seacock bases. I did not go through the hull with the bolts, just into the backing plate.
I have been dreading doing the three in the head, but based on my experience with the 4 aft, I will probably just take off the ball valves and screw on seacocks. There was nothing wrong with my through hulls under the sink, even though two showed a little red on the outside. If the threads are messed up from the ball valves I'll find a way to clean them up.
I checked out your blog - very nice. We have the same interior it looks like. Good luck with your project!
