So on the sew-through thing I've been thinking circular arcs. Given a base
(diameter) b the cross-sectional area is b*pi . Depending on b (the width of a baffle chamber) b*pi may be too small for the area you need for the amount of down you'll stuff into the chamber. For this then I approximate the chamber cross-section as a rectangle with base b and height h, such that the cross-sectional area b*h + b*pi is equal to what I've computed I need for the baffle chamber. That's for small b of course (narrow chambers). For larger ones I figure the base b is a chord on a circle and work from there. Like you, when I have the shape of the curve of the outside fabric, I compute the arc length.
Well that's theory anyway. I'm heading out to the lab right now....