Do you ever get tired of having your duvet (stupid name btw) crumple up or fold inside the cover resulting in having cover-only ends? It's like eating a sausage that's only skin. This may be a symptom of size mismatch between duvet and cover, but it happens anyway too. Covers with zippers are better and covers with buttons or just straight up a hole are worse.

Option 1

Why not have a cover that has two top and bottom parts that you can zip together all around, zipping in duvet together? You could have a duvet with a left zipper side all around it, a bottom part of cover with two zippers - right side to merge with duvet, left side to merge with top part of cover, and a top part with right side zipper to merge with bottom part of cover. There are obvious benefits and drawbacks to this

• Very easy to switch covers
• Bottom part and top part of cover is now seperated, so you can do things separately to top and bottom
• You can mix and match duvet and cover sizes because excess of either of two will just fold onto itself somewhere (definetely not in the corners!)
• You will no longer experience sleeping under a thin sausage skin
• There are two zipper lanes all around your duvet-cover combo which might be unconfortable to sleep with
• You need a specialized duvet specifically for this design

With this option, you don't even need the top part of cover as long as you will get used to always sleep "correctly". This would also eliminate the need of second zipper lane.

Option 2

To partly solve the first drawback of option 1, you can do the same, but leave the top part of your duvet-cover combo as is (the part that covers your shoulders and that you sometimes hold with your hands). You will still need to insert the duvet into the top part of cover as usual and then zip-in the rest, but it should be easy because you should see everything whats going on inside and the initial problem of folding or crumpling shouldn't occur either because 80%~ of other parts are managed.

• Easy to switch covers
• You can mix and match duvet and cover sizes because excess of either of two will just fold onto itself somewhere (definetely not in the corners!)
• You will no longer experience sleeping under a thin sausage skin
• There are two zipper lanes all around your duvet-cover combo which might be slightly unconfortable to sleep with. You can still feel it with your legs of body core
• You need a specialized duvet specifically for this design

Option 3

We can also just add a couple of small buttons at corners of the whole thing to keep duvet and cover together. Maybe also two buttons in the middle of long edges?

• Can be pretty easily adapted to existing duvets and covers
• You can mix and match duvet and cover sizes because excess of either of two will just fold onto itself somewhere (definetely not in the corners!)
• You will probably no longer experience sleeping under a thin sausage skin
• Slightly harder to switch covers compared to usual