Well done! I especially like your finishing touch of the blocks at the bottom edges.
Helen S, if you sew the strips directly on the down comforter you'll lose some of the loft and warmth of the comforter, but you could make a duvet cover (like a giant pillowcase), adding your extra width/length, and then tack or tie it in place. You could even add ties to both the comforter and cover so it would stay in place but could be removed for laundering.