you can simply stitch the cover to the duvet with embroidery thread. if you use big stitches, they're easily removed when it's time for washing.

a few of my duvet covers shift and this was the semi-permanent solution. not all of them shift, and sometimes clocking the duvet 180deg inside helped.

down duvets are lighter and fluffier and gave me less problems. we don't tie anything anymore, and the 'problem' covers are relegated to the bottom of the pile and don't see much use anymore.