Stacking filters and using an undersized lenshood will certainly produce vignetting, but it tends to be rather harsh, unpleasant, "hard" in nature. What could be used to provide a more subtle effect?

Perhaps something like a graduated neutral density filter with a large, oval, clear centre spot and a graduated increase in density from the centre outwards.

Is such a thing available? How could one manufacture one DIY?

Maybe smearing an UV filter with tinted petroleum jelly would work?