In my local supermarket I found the solution (which wasn't genious, but I hadn't thought of it before):
A slip with a pretty edge and adjustable bra straps. By lengthening/shortening the straps, I could make the extender fit skirts and dresses of various lengths.
And it was a lovely soft viscose (rayon) and on sale for 12 kr. (2$).
It was a size XXL, so I took it into my size.
It was also rather long, too long to work as skirt extender (that is sort of ironic). So I removed 15 cm (6") of the length.
Blogpost: Dress and skirt extender
And action. Here I show it being almost as short as it can be:
When the straps are extended in their full length, the bottom of the lace hits just above my knees.
And the neckline is below my bust.... But it sits surprisingly comfortably there and my clothes cover it.
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