I’m finished with the cording of my Regency stays, and have sewn one of the back pieces to the front. I use the method used on the original I base mine on. It’s done like this:
Fold the seam allowances of the outer fabric and the lining so they are between the layers of fabrics. Put the front and back together right side to right side and stitch through the two double layers of outer fabric and the double layers of the lining.
And back through the outer layers of fabrics and the lining.
The seam will end up looking slightly wavy, and is pretty sturdy, so it will be almost like a light boning.
Unfortunately the temporary stitching I made for fitting purposes has left horrid marks on the outer fabric. Hopefully they’ll go away in time.
This kind of seam was used in bodices during the 18th century, and is rather nice to use. One seam and you get a neat result, both on the outside and inside.