It's Just the Beginning
The previous examples barely scratch the surface of possible apertures to control or manipulate SA for artistic purposes. The basic ingredients are not complicated. Start with a simple positive lens with a healthy amount of SA (see my Homemade Lens Primer for a refresher on that) and add an aperture that limits the amount of light passing through the outer part of the lens. You can start with the example disks I've shown or experiment with your own designs.
I'll leave you with this last example image shot with a homemade soft focus lenses.