I'm pretty certain - but not absolutely so - that the first image below is taken with a "black scarf" filter and the second without.
and
You can see (at least, IMO) a very slight difference in the skin texture - somewhat reduced in the first image. A heavier mesh makes more diffusion. As I recall, I tried several different colors with little or no visible effect, but it's been quite a while.
The black scarf filter is essentially a black mesh, like a black nylon stocking. The people at Garage Glamour popularized a particular black nylon scarf from Walmart.
You can make a permanent filter by "clamping" the mesh between two step-up adapters, like 49 to 52 and 52 to 55 mm. That's how I did it.
As for marking a UV filter, I believe you'll find that the pattern could be either random or regular, with only minor differences. The key would be making very small marks, sufficiently closely spaced to ensure slight diffusion of the image.
BTW, there is a difference between using a filter and digitally softening. A filter imparts a somewhat more random softening. I suppose a skilled Gimp user can vary the amount of softening selectively, but even that isn't quite the same.
Good luck