How to change root viewport canvas cull mask in 2D

Godot Version

v4.1.2.stable.official [399c9dc39]


I am trying to follow this tutorial to implement water shader ripple effects:

The tutorial is for Godot 3D so I am trying to translate it to 2D, and I run into an issue when he starts using cull masks. In 3D cull masks are handled by cameras, but in 2D it seems they have to be set up using viewports instead. But the root viewport is not accessible in the editor as far as I can tell.

So for example, if I want to have a sprite in my 2D scene which is visible to a viewport but invisible to the main camera, how would I accomplish that?