Cut out shader with geometry depth and alpha

Hello, I'm trying to do a shader like shown on image: alt text The shader I've found on the forums cuts out everything on camera till background. But I would like it to cut only the depth of the box that have the material applied. So I want to see a geometry beneath. But also be able to apply the masking texture. Could you please help me with the shader that would do what I would like? I'm totally noob in shader language but I guess it's an easy thing for somebody who knows them well :).

Here is the shader:

  Shader "Alpha Test Depth Mask" {
       
      Properties {
          _MainTex ("Texture", 2D) = ""
      }
       
      Category {
          Tags {Queue = Background}
          ColorMask 0
       
          SubShader {Pass {
              GLSLPROGRAM
              varying lowp vec2 uv;
       
              #ifdef VERTEX
              void main() {
                  gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
                  uv = gl_MultiTexCoord0.xy;
              }
              #endif
       
              #ifdef FRAGMENT
              uniform lowp sampler2D _MainTex;
              void main() {
                  if (texture2D(_MainTex, uv).a > 0.99) 
                      discard;
              }
              #endif      
              ENDGLSL 
          }}
      }
       
      }