When I made my climashield quilt (5oz), I made it tapered to save weight and I originally had a drawstring foot. Then tried a zipped and velcroed foot...
Then, I finally decided this was too tight for my size 13 feet, and retrofitted a circular panel.
Having tested the quilt, I also found that 40*F was a low as I'd be comfortable to use it, so I figured that by sewing the foot panel on afterwards having an almost 'stitched through' seam would not be too bad heat-loss wise. By stitching the already sewn edges together , it means that there is a sort of thicker multiple layer of seam allowance which acts as a 'baffle' at the join.
(Obviously, the Finished circumference of circle is same/close to as the length of end edge of main quilt)
To attach it, I pinned it, and started sewing at the centre of the quilt and worked both directions in turn around to the back, then sewed the long edge up some way to create a permanent foot box.
As I made the footbox fairly big, it's also worked out well as a good synthetic overquilt for a lighter down bag.