A quick update. In my OP, I trimmed the center piece a bit too narrow so I gave it another try. A perfect fit allows the paper to stay in place without any shimming. Pressing against the web of the rail is enough to hold it in place.
I also updated the texture. I downloaded some concrete textures and used them to enhance the original texture that I made in Blender. I loaded the concrete photos into new layers and used various merge modes (Overlay, Hard Light, and Luminance.) You may need to adjust the Levels and Opacity of these layers to keep everything within the same tonal range.
I also used the Emboss filter to paint in some chips along the edges of the center piece.
Steve S