This is how I did solve the problem with my too narrow Dutch Down Duvets and its Cinderella Duvet Covers...
At TJ Maxx we found a nice white all cotton queen sheet set. As I could use the fitted sheet for our bed, the flat sheet got used for adding 16 inch (or 40 cm) of width to the duvet covers.
Here it is, added on already!
The hard part was the tedious opening up of the seam with the seam ripper.
Once that got done it was easy.
So a total of 32 inches of white cotton fabric for each duvet cover was added on.
To make it real easy, I even kept the flat sheet's folded hem and only added ties for closing at the bottom by stitching white bias tape together.
Had to mark those spots with my pink chalk (still visible here).
Simple and low cost. Learn from Mariette...
Related link:{Dutch Twin Down Duvets Altered to American Size} | previous post by me with chart about size differences between Dutch & American Down Duvets...