It fits OK when first put in, then it slowly slides down to the bottom, leaving about 4-5 inches of empty duvet fabric at the top. This does not suit my somewhat OCD self.
bunchy bottom and flat top
So I took out the comforter, turned the duvet inside out, and lay the comforter back on top.
With everything in place and pulled as tight as I could make it, I began to pin the corners of the comforter to the duvet. From seam to seam so hopefully no holes and loose feathers or whatever is in my comforter.
Then for good measure, and so the top wouldn't sage in the middle, I added two safety pins to the top.
I had already safety pinned the mattress pad to the mattress (yes I am OCD about the mattress pad bunching up under the sheets too) and it works well. I hope this is a good solution for the duvet too.
So far so good.