As explained above, the reason for the petal-shaped hood is that wider-angle lenses can't use a round hood as they would cause vignetting at the corners. So, they shorten the corners to eliminate it.
On mid- and longer telephotos (and zooms), the narrower angle of view provided by the lens permits a longer, fixed-length (eg, cylindrical) shape without vignetting. Per the Canon website for the 85 1.2, its angle of view is 28 degrees 30 minutes, and at the length needed for best protection, the cylindrical hood works just fine. So, to save manufacturing costs, that's what they use.
I bought the 'genuine' Canon hood when I bought my 85 1.8 at B&H. Worked fine, although a bit pricey for a simple piece of plastic. Although someone above suggested get a cheapo flea-bay hood and modify it with a dremel cutting tool, you'd effectively be reducing the level of angular incident light protection the hood is there to prevent. Bump and ding protection would also be slightly reduced as well.