If I understand you correctly, you will quilt a plain muslin sandwich and then put that into a duvet cover that you'll make? And then sew in the ditch of the cover to attach it to the muslin sandwich? A friend of mine used the wool batting and it really puffed up like a comforter because she hadn't quilted it closely enough, so I see this happening to you unless you quilt the sandwich very closely. Then you also have the entire center of your cover loose and unattached.