Cutting holes through UI images in Unity using stencil buffers and shaders

Unity Engine
1

I am quite new to shaders in unity and have a very basic understanding.
I have a foreground image on the UI Layer that covers the whole scene. How can I cut through this foreground image and show a small part of the 3D worldspace behind? I need to make multiple holes dynamically and not just a simple static hole.

I have read that I need two shaders, one for a mask (x-ray) and one for the mask target. I thought it would be a good idea to take a lightweight shader for a base, so I took an Unlit Transparent shader.

Shader "Custom/HoleShader"
{
    Properties
    {
        _MainTex ("Base (RGB), Alpha (A)", 2D) = "white" {}
        _Color ("Tint", Color) = (1,1,1,1)

        _StencilComp ("Stencil Comparison", Float) = 8
        _Stencil ("Stencil ID", Float) = 0
        _StencilOp ("Stencil Operation", Float) = 0
        _StencilWriteMask ("Stencil Write Mask", Float) = 255
        _StencilReadMask ("Stencil Read Mask", Float) = 255

        _ColorMask ("Color Mask", Float) = 15

        [Toggle(UNITY_UI_ALPHACLIP)] _UseUIAlphaClip ("Use Alpha Clip", Float) = 0
    }
    SubShader
    {
        Tags { "Queue"="Geometry-1" }

        ColorMask 0
        ZWrite Off

        Stencil {
             Ref 1
             Comp always
             Pass replace
        }
    }
    FallBack "UI/Default"
}

Then I created another shader for the mask target (in my case for the UI image on the UI Layer and for 3D elements in the worldspace space that are not supposed to be seen through the hole/window).

Shader "Custom/HoleTargetShader"
{
    Properties
    {
        _MainTex ("Base (RGB), Alpha (A)", 2D) = "white" {}
        _Color ("Tint", Color) = (1,1,1,1)

        _StencilComp ("Stencil Comparison", Float) = 8
        _Stencil ("Stencil ID", Float) = 0
        _StencilOp ("Stencil Operation", Float) = 0
        _StencilWriteMask ("Stencil Write Mask", Float) = 255
        _StencilReadMask ("Stencil Read Mask", Float) = 255

        _ColorMask ("Color Mask", Float) = 15

        [Toggle(UNITY_UI_ALPHACLIP)] _UseUIAlphaClip ("Use Alpha Clip", Float) = 0
    }
    SubShader
    {
        Tags { "Queue"="Geometry" }

        Stencil {
             Ref 1
             Comp notequal
             Pass keep
        }
    }
    FallBack "UI/Default"
}

Would be very thankful for any help!

2

Without testing, you most likely need to capture the mask first in a Command Buffer then pass that to a custom texture that the shader can read. If you can get the stencil up first its the same thing I think.
Unity - Scripting API: Shader.SetGlobalTexture (unity3d.com)
I have an old post that is not rock solid. There are no great examples or docs for doing this stuff really since each need is a bit differnt
How to generate Depth Buffers in a CommandBuffer? - Unity Forum