Creating a shader with "local" screenspace uv?

I'm trying to create a shader that creates a local screenspace uv. You can see my code at the bottom, my guess is that my method of trying to create a "bounding rect" is wrong. Below you can see the current result which is just a screenspace uv.

The effect I'm moving towards is a variation of the unmoving plaid, sort off, cause it'll clearly be moving but you know. However i'd like to make it look more like this;

 // Upgrade NOTE: replaced 'mul(UNITY_MATRIX_MVP,*)' with 'UnityObjectToClipPos(*)'
 
 Shader "Custom/Gradient" {
  
    Properties {
      
  
     }
       SubShader {
    
         Tags{"Queue"="Transparent"}
  
         Blend SrcAlpha OneMinusSrcAlpha
         Lighting On
         ZWrite On
      
     Pass { //pass1
         CGPROGRAM
         #pragma vertex vert
         #pragma fragment frag
         #include "UnityCG.cginc"
      
         struct v2f 
         {
               float4 pos : SV_POSITION;
               float4 scpos : TEXCOORD0;
               float4 screenRect : TEXCOORD1;
         };
    
         v2f vert (appdata_full v)
         {
             v2f o;
             o.pos = UnityObjectToClipPos (v.vertex );
             o.scpos = ComputeScreenPos(o.pos);
 
             fixed2 corn1 = ComputeScreenPos(fixed4(0,0,0,1));
             fixed2 corn2 = ComputeScreenPos(fixed4(1,0,0,1));
             fixed2 corn3 = ComputeScreenPos(fixed4(0,1,0,1));
             fixed2 corn4 = ComputeScreenPos(fixed4(1,1,0,1));
 
             fixed2 corn5 = ComputeScreenPos(fixed4(0,0,1,1));
             fixed2 corn6 = ComputeScreenPos(fixed4(1,0,1,1));
             fixed2 corn7 = ComputeScreenPos(fixed4(0,1,1,1));
             fixed2 corn8 = ComputeScreenPos(fixed4(1,1,1,1));
 
             fixed minX = min(corn1.x,min(corn2.x, min(corn3.x, min(corn4.x, min(corn5.x, min(corn6.x,min(corn7.x,corn8.x)))))));
             fixed minY = min(corn1.y,min(corn2.y, min(corn3.y, min(corn4.y, min(corn5.y, min(corn6.y,min(corn7.y,corn8.y)))))));
 
             fixed maxX = max(corn1.x,max(corn2.x, max(corn3.x, max(corn4.x, max(corn5.x, max(corn6.x,max(corn7.x,corn8.x)))))));
             fixed maxY = max(corn1.y,max(corn2.y, max(corn3.y, max(corn4.y, max(corn5.y, max(corn6.y,max(corn7.y,corn8.y)))))));
             o.screenRect = fixed4(minX, minY, maxX, maxY);
             return o;
         }
         fixed Vnormalize( fixed x, fixed amin,  fixed amax)
         {
             return (x-amin)/(amax-amin);
         }
      
         fixed4 frag (v2f i) : COLOR
         {
             float2 uv2 = (i.scpos.xy/i.scpos.w);
             float2 uv = float2(Vnormalize(uv2.x, i.screenRect.x, i.screenRect.z), Vnormalize(uv2.y, i.screenRect.y, i.screenRect.w));
 
             fixed4 col = fixed4(uv.xy, 1,1);
  
             return col;
         }
              
         ENDCG
     }//pass 1
   } //subshader
 }//shader