If I could comprehend what you are describing I'm sure I would be able to help you. Do you want to change the shape of the corners? Do you want to sew buttons to attach the duvet to a cover? Do you want to have it fit like a fitted

sheet? In the years I worked with custom made items I never saw anything like this photo that is a ready made.

I want to change the comforter's corners at the foot of the bed to look like the corners in the picture you posted (like a fitted sheet). You're right; fitted comforters are not ready made!

Hmmm, I was so sure I had seen it all in the custom workroom I used. But no, no I have never heard of, let alone seen, a comforter fitted at the ends. And I saw a LOT! Best of luck to you .

I made a removable cover for a rectangular ottoman that is fitted on all four corners like that. I did it by making simple darts on all four corners, then trimmimg the fabric down. It might be more difficult to accomplish this considering the bulk of the comforter, but if you are using a thinner more quilt like cover, I think it is certainly possible.

It looks as if you simply square, box out, the corners. It looks that this one in the photo does not tuck back under the mattress like the corner of a fitted sheet. It looks like a simple squared box corner.

If you want it actually be used like a fitted sheet you will have to make just like a fitted sheet and be tucked back under the mattress or else it will not stay in place when you are in the bed.

This picture looks to me that is a simple squared corner that is more for show than for use.

I have seen these kinds of fitted covers around.

Unless it is make like a fitted sheet corner, it is for show and not for practical use.