Turning a found material shader into a post-processing effect?

I am working on a project involving selectively masking one camera overlaid on another based on alpha values. I found a shader that works well for this purpose when applied to a material on a quad placed directly over one camera. By rendering out both a 2nd camera and an 'alpha' camera to RenderTextures, I am able to achieve the effect I want with this shader on the quad.

Demo here

Here is the shader in question:

 Shader "Custom/CustomFadeShader" {
     Properties {
         _Cutoff ("Cutoff", Range (0,1)) = 0.5
         _CutoffRange ("Cutoff Range", Range (0,1)) = 0.1
         _MainTex ("Base (RGB)", 2D) = "white" {} // "Light" camera RenderTexture
         _AlphaTex ("Mask", 2D) = "white" {} // "Alpha" camera RenderTexture
     }
     SubShader {
         Tags {"Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent"}
         Blend SrcAlpha OneMinusSrcAlpha
         LOD 200
         Lighting OFF
        
         CGPROGRAM
         #pragma surface surf Unlit alpha noambient novertexlights nodirlightmap nolightmap noforwardadd
  
         half4 LightingUnlit (SurfaceOutput s, half3 dir, half atten) {
             fixed4 c;
             c.rgb = s.Albedo;
             c.a = s.Alpha;
             return c;
         }
  
         sampler2D _MainTex;
         sampler2D _AlphaTex;
         half _Cutoff;
         half _CutoffRange;
  
         struct Input {
             float2 uv_MainTex;
         };
  
         void surf (Input IN, inout SurfaceOutput o) {
             half4 c = tex2D (_MainTex, IN.uv_MainTex);
             half d = tex2D (_AlphaTex, IN.uv_MainTex).r;
             o.Albedo = c.rgb;
  
             half r = _Cutoff * (1 + _CutoffRange * 2) - _CutoffRange;
             o.Alpha = (d - r) * (1 / (_CutoffRange));
         }
         ENDCG
     }
     Fallback "Diffuse"
 }
 

What I'm trying to figure out is if there is a better way to implement this effect as a post-processing effect on a camera rather than combining RenderTextures over a quad? I am a total novice when it comes to shaders; any direction about how to translate this material shader would be appreciated!