Export a Master File of a matte

Was in a hurry today and couldn't figure out how to do this: I wanted to export a master file of my project's "matte" (alpha channel).

I know how to export the project in 4444 with the embedded alpha, but I need it as a separate file. I know I can change my viewer to "alpha" and play it out, but I needed it on disk.

Do you mean export a grayscale matte channel? Put a white generator on top and apply blend mode silhouette alpha or maybe it's stencil. Can never remember which is which.