I wrote this last night:
I hadn't though of using an Opacity Mask and I came up with Marco's approach.Yes, it would be useful to have a viewing.
You could create a Scatter Brush along the lines of Novelty > Coarse Spray and make the Line quite wide.
I have no idea if you want a box shape but this handles any shape as well as allowing Transparency of the spray.
I stopped at that point as once you Subtract, you are stuck with a fixed shape.
Thank you both @PhilM & @handrawn as your approach retains the means to edit and experiment.
Another arrow for my quiver.
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