Mask Sprite With Another Sprite

I have 3 GameObjects using a SpriteRenderer to draw solid-colored quads behind a larger, transparent one as shown in the attached screenshot. In this case, the larger, transparent image is the rocket. The rocket image is also using a SpriteRenderer to draw the image. I want to mask my 3 large quads using that rocket image, but my knowledge of how to blend are simple. I have 4 SpriteRenderers on the same sorting layer (Default) where the 3 solid-color SpriteRenderers have an "Order in Layer" of 1, and the rocket SpriteRenderer has an "Order in Layer" of 2. I want to achieve the same effect as UI Mask, except with the SpriteRenderer. I'm pretty sure I have to create a custom shader, and attach it to each solid-colored SpriteRenderer, and reference the rocket's SpriteRenderer using the sorting layer (probably using "Queue" in the shader?)

I'm really not sure where to go with this, but I did come across this thread earlier on blending. It's a fixed-function shader, so it'll have compatibility issues in Unity 5. I'm trying to convert it over to a regular vertex/fragment shader pair. Does anyone have any ideas on how to mask those quad sprites against the shape of the rocket sprite?

EDIT: I forgot to mention that I went to this page to figure out what kind of blending I need to use. It's for OpenGL, but Unity's renderer runs on top of APIs like OGL, DX, etc. The blending mode I think I need is SrcAlpha, SrcAlpha, but no luck in my shader.

Here's the example shader I've been trying to put together:

Code (CSharp):

1. Shader "Custom/SpriteMask"
2. {
3.     Properties
4.     {
5.         _MainTex ("Base (RGB) Alpha (A)", 2D) = "white" {}
6.         _Cutoff ("Base Alpha cutoff", Range (0.0, 0.9)) = 0.5
7.     }
8.  
9.     SubShader
10.     {
11.         Tags {"Queue" = "Transparent+1"}
12.         Offset 0, -1
13.         ColorMask 0
14.         ZWrite On
15.         Blend SrcAlpha SrcAlpha
16.  
17.         Pass
18.         {
19.             AlphaTest Greater [_Cutoff]
20.             SetTexture [_MainTex]
21.             {
22.                 combine texture * primary, texture
23.             }
24.         }
25.     }
26. }
27.  

It's fixed-function (should convert to vert/frag), but it's what the first linked post had. Since it's the closest thing I could find to a solution, I was going to keep it like that until I'm more familiar with CG as it's pretty different from GLSL.