It would be horrible to finish this corselet then wish I'd used a different type of boning. Horrible. So I did a test. The top casing would hold plastic, the middle spiral steel, and the bottom flat steel.
The original pattern showed the bones to be only 4 inches across. I had a piece of 5 inch flat steel that was already cut and finished at both ends so I made all test pieces 5 inches. After they were slipped in place I tried the corselet on.
Spirals right? I hate the way the ends of the flat steel poke outward. That's not pretty. I'm thinking of adding one more row of boning at the bottom. I may wait until the garters are attached to see how the fit is when anchored down though.
Look at that straight 1920's silhouette!