How to render object index as alpha?

I am currently working on a composition which includes an open door containing a room to be layered over a background (I guess a bit like the Tardis?). Here’s some pictures of the scene:

I have set the object index of the front wall to 1, and included an object index pass so that I can use the object index pass as the factor for a mix node. With this, I can turn the front wall to any colour etc, but I don’t know how to make it render as pure alpha, so that all is rendered is the door and the view through the door of the room inside. Here’s what I currently have:

And a link to my node setup:

So how do I make that green (checkerboard) alpha? Could this involve layers or masks? Any help would be greatly appreciated.


Thanks a lot- just what I was looking for :slight_smile: