I'm working on a wooden fence, similar to the picture below. It has a 2"x4" wood rail that will be the length of all four 4"x4" pressure-treated wood posts. This rail will be attached to the posts with screws.
Since the concrete where the posts are attached to is uneven, there will be "empty" space between rail and some posts in-between.
What can I use to fill that unwanted space that's strong enough to insert screws? I don't have the tools to cut the 4"x4" posts to the correct height, so that's out of the question.