Masking a Static Texture

You might have a look here.

stenciling if you just need a mask im not to practiced with using the stencil buffer myself.

using a shader this shader maybe overkill here because there is also doing transparency and blending but i obviously favor using shaders.
However a full example project is included, its old you might have to rebuild it into a new project.

There are quite a few posts to be found on both if you click on the search icon in the top right of this page.