What you’re trying to recreate requires a bit of trickery… mainly because diffraction gratings require both incoming and outgoing ray vectors to compute correctly (and we can only access the incoming ray).
Anyway, here’s a setup that you could use based on an old shader I built a few years ago. It’s neither perfect nor physically correct, but it’s controllable.
(It may require more samples to clean up since it’s faking anisotropic reflections)
