I built a power rack. After about a year of using it, I love it more than when I finished. I believe it will outlive me.
I recently built a roman chair out of 2x4's. I also recently built a back extension, but it doesn't fit me well so I have to revise the design.
Whatever you build make sure you do a lot of drawings first. Obviously, using something like AutoCAD is going to REALLY help (I used it for the power rack). Otherwise hand drawings to scale will work. The point here is to notice any design/ergonomic/ease of use flaws that may be present.
Why do you ask about building a competition bench specifically? Are you a bench specialist who can't afford a Forza? Do you squat? why not build a power rack?
EDIT: a glute ham raise would be very easy to build.