Out-of-focus highlights of any shape may be created by making a simple black card lens hood that fits snugly around the front of a particular lens. Just cut the required shape out of a piece a thin black card and fold the edges around the barrel of the lens. The final "lens hood/filter" should be a reasonably good fit and the aperture should be as close as possible to the centre of the lens. The size of the aperture is found by experiment and depends upon the size of the lens front element.
In the image below, a star-shaped aperture has been created in a simple sheet of thin black card and attached to the front of a Nikon 85mm f/1.8 prime lens with masking tape. The points of the star are about one inch in diameter. With the star aperture fitted, the lens should still be set to the largest possible aperture (wide open).