How to render an object buffer of hollow space?

Ok there is probably a very simple solution to this:

But I have made a gas chamber frame and would like to render out an object buffer of all the inbetween bits of the frame. So basically, where the gas will be.

I made a rectanglur model that fits inside the chamber perfectly and applied an object buffer to that, but if I turn it off so you can see the inside edges of the gas chamber, the object buffer doesnt render out.

Is the only option to run the render a second time with the cube in, and do the luma matting later in after effects. Basically I just wanted to do 1 render rather than 2?

Any thoughts.