How to set opacity of a mesh with post process

Hello,

I’m making a toon shader but i have problems with the outlines PP material.

Without masking the skybox there are lines in it so I don’t want that and when it’s masked the objects with the sky as background aren’t outlined anymore.

So I placed a sphere (with inverted normals) and now the outlines are good evrywhere and the skybox is clean but the problem is i need to set the opacity or at least hide this sphere where the PP mask is (in green on the second image).

Can someone help me please ?

Thanks.