Nothing resonates more with steampunk than brass. Made from brass blocks, brass backdrop is one of the more common and easily-obtained backdrops. Place this backdrop in sets of four as a square and you will see the large panel of riveted brass.
How to Obtain
This backdrop can be crafted or traded for, and is also found in various world-generated structures, where it can be mined.
If you craft this block while standing near a working forge, you'll get twice as much back as normal.