help with fade/cutout skybox shader

Hi guys I'm trying to give a fade/cutout to this shader, this is not my shader and I found that on the unity forum. I have tried but with no luck, the "best" result is this with the clip() function.. but sadly in the reverse like this and the hard edge of the cut out:

my ideal result of the shader is this, with a cubemap and an Alpha channel with fade:

here the original code:

 Shader "Skybox/CubemapTransparent" {
 Properties {
     _Tint ("Tint Color", Color) = (.5, .5, .5, .5)
     [Gamma] _Exposure ("Exposure", Range(0, 8)) = 1.0
     _Rotation ("Rotation", Range(0, 360)) = 0
     [NoScaleOffset] _Tex ("Cubemap   (HDR)", Cube) = "grey" {}
 }
  
 SubShader {
     Tags { "Queue"="Background" "RenderType"="Background" "PreviewType"="Skybox" }
     Cull Off ZWrite Off
     Blend SrcAlpha OneMinusSrcAlpha
  
     Pass {
      
         CGPROGRAM
         #pragma vertex vert
         #pragma fragment frag
  
         #include "UnityCG.cginc"
  
         samplerCUBE _Tex;
         half4 _Tex_HDR;
         half4 _Tint;
         half _Exposure;
         float _Rotation;
  
         float4 RotateAroundYInDegrees (float4 vertex, float degrees)
         {
             float alpha = degrees * UNITY_PI / 180.0;
             float sina, cosa;
             sincos(alpha, sina, cosa);
             float2x2 m = float2x2(cosa, -sina, sina, cosa);
             return float4(mul(m, vertex.xz), vertex.yw).xzyw;
         }
      
         struct appdata_t {
             float4 vertex : POSITION;
         };
  
         struct v2f {
             float4 vertex : SV_POSITION;
             float3 texcoord : TEXCOORD0;
         };
  
         v2f vert (appdata_t v)
         {
             v2f o;
             o.vertex = mul(UNITY_MATRIX_MVP, RotateAroundYInDegrees(v.vertex, _Rotation));
             o.texcoord = v.vertex;
             return o;
         }
  
         fixed4 frag (v2f i) : SV_Target
         {
             half4 tex = texCUBE (_Tex, i.texcoord);
             half3 c = DecodeHDR (tex, _Tex_HDR);
             c = c * _Tint.rgb * unity_ColorSpaceDouble;
             c *= _Exposure;
             return half4(c, 0.15);
         }
         ENDCG
     }
 }  
 } 

someone can help me? thanks

p.s. here the clip() code:

 Shader "Skybox/CubemapTransparent2" {
 Properties {
     _Tint ("Tint Color", Color) = (.5, .5, .5, .5)
     [Gamma] _Exposure ("Exposure", Range(0, 8)) = 1.0
     _Rotation ("Rotation", Range(0, 360)) = 0
     [NoScaleOffset] _Tex ("Cubemap   (HDR)", Cube) = "grey" {}
     _alpetto ("alpetto", Range(0, 1)) = 0
 
     _CutoutThresh("Cutout Threshold", Range(0.0,1.0)) = 0.2
 }
  
 SubShader {
     Tags { "Queue"="Background" "Queue"="Transparent"  "RenderType"="Background" "PreviewType"="Skybox" }
     Cull Off ZWrite Off
     Blend SrcAlpha OneMinusSrcAlpha
  
     Pass {
      
         CGPROGRAM
         #pragma vertex vert
         #pragma fragment frag surf alpha
  
         #include "UnityCG.cginc"
          float _CutoutThresh;
         samplerCUBE _Tex;
         half4 _Tex_HDR;
         half4 _Tint;
         half _Exposure;
         float _Rotation;
         float _alpetto;
 
  
         float4 RotateAroundYInDegrees (float4 vertex, float degrees)
         {
             float alpha = degrees * UNITY_PI / 180.0;
             float sina, cosa;
             sincos(alpha, sina, cosa);
             float2x2 m = float2x2(cosa, -sina, sina, cosa);
             return float4(mul(m, vertex.xz), vertex.yw).xzyw;
         }
      
         struct appdata_t {
             float4 vertex : POSITION;
         };
  
         struct v2f {
             float4 vertex : SV_POSITION;
             float3 texcoord : TEXCOORD0;
            // float4 uv_Tex;
         };
 
          struct Input {
          float2 uv_Tex;
      };
  
         v2f vert (appdata_t v)
         {
             v2f o;
             o.vertex = mul(UNITY_MATRIX_MVP, RotateAroundYInDegrees(v.vertex, _Rotation));
             o.texcoord = v.vertex;
 
             return o;
         }
  
         fixed4 frag (v2f i) : SV_Target
         {
             half4 tex = texCUBE (_Tex, i.texcoord);
          
            half3 c = DecodeHDR (tex, _Tex_HDR);
          
             c = c * _Tint.rgb * unity_ColorSpaceDouble;
             c *= _Exposure;
             clip(c - _CutoutThresh);
 
            
             return half4(c, _alpetto);
           
 
         }
 
              
          
 
         ENDCG
     }
 }  
 }