When you have quarter inch seems and intend to wash something often, I think it's essential to have something reinforcing your piecing lines.
What if you put the pieced top on a layer of flannel and did some quilting-- no batting, it's totally up to you how you quilt it-- stitch in the ditch, meander, or all out. you could even do some trapunto if you want to have a little bit of texture and puffiness without adding too much weight. Then you create the duvet/pillow case like structure complete with a backing (seems like a flat sheet would be perfect.)