Shader to render RenderTexture plus alpha?

Hi all,

my MiniMap has a circular shape, the RenderTexture (naturally) doesn't. I would like to get rid of the unneeded corners. Is a shader which will take the RenderTexture as unlit and add a greyscale texture as alpha channel? If so, has anybody a clue how this could be done? Or are there any better ways to achieve the same?

Thanks for the help!