The J.P. Ryan patterns look great.. thanks for the reference!

Oh, no no no no. Panniers don't work that way. Here is an example of large panniers in the Metropolitan: www.metmuseum.org/toah/hd/e...3.65.2.htm

Note how short they are, and how they most certainly don't have vertical bones. In fact, no skirt structure throughout the entirety of steel history has been made that way, except with very short vertical ones. Farthingales don't and didn't have vertical bones. And the first picture you posted was of a large supportasse for her ruff.

At any rate, panniers rely on the cut of the fabric and the tension of rings of steel to create that impressive silhouette. So you take the width of what you want the panniers to be, and the width of your body from ass to belly button, a rather comfortable measurement. Do a perimeter based on those, and cut a length of fabric that is the perimeter, plus seam allowance. Cut an oval (or square, depending) of the same perimeter plus seam allowance, and a hole in the middle the circumference of your natural waist (3" below belly button) and slit from there to short edge. Stitch long perimeter fabric around oval. Sew the perimeter shut but not the oval. bind the slit and put binding around hole in oval for a draw string. Stitch about 3 rows of bias tape around the perimeter bit of fabric. Use as casing for boning, which could be spring steel or wicker cane. Wicker cane being more period. Stitch ties on the inside on the long edges so that you can tie them together for better holding of a flat plane in the front and back.

This pattern doesn't allow for pockets. If you want pockets underneath, make slits instead part way down each long oval end, and bind them. When you make the waistband, let one side close and leave the other side open to tie.