I found when I installed our extend-a-rod that the wood backing was very small. I took my best guess. At one end it worked out great. The other end - well, someday I'll be working on that, but for today, it's workin'.
It amazes me how chintzy these trailer guys can be with simple things like wood backers. They probably throw chunks of wood like that into the garbage every day, so it can't be an issue of cost.
Jim