The cut of the fabric creates the shape, and the boning supports the fabric so it can maintain that shape. This corset is not designed to be a dramatic figure changer, I think plastic boning will suffice. Not all plastic boning is created equally though. There is decent stuff and crap stuff. Under stress the cheap plastic can warp and collapse. The good plastic is made in Germany and is easy to spot because it has white lines running its length.
Plus, I used plastic for the mock-up, now that I've got all those short pieces it would be nice to put them to work. I slid them in the openings I left along the bottom edge, three to each side, and one along each side of the grommets.
I stitched across the openings to sew them closed, and that's it. The construction is finished.