I partially agree with the above - plywood is better than solid wood. But the backing plate, whatever it is made of, should be on the underside of the deck, not the liner as there is a gap between the liner and the deck. Tightening the bolts will move the liner closer to the deck. I would try to get the backing block right under the deck.
As far as epoxying the backing block to the deck, or the inside of the hull in the case of a through hull, I think it is the best idea as it will become part of the hull or deck, strengthening it a great deal.
For through hull installation, see this link: Replacing Thru-Hulls and Seacocks Photo Gallery by Compass Marine at pbase.com
And fiberglass or G10 is ideal. What I do is save any fiberglass scraps I remove from any boat - mine or others as I work on boats - and cut backers from them for my use. Any boatbuilder should have offcuts as well, probably free for the asking.