If you look close in this picture, you can see where the opening has been cut just to the right of the reflector on the light. I put the boards back up temporarily.
Here's the pass-thru under construction. It's about 18 inches between walls, so I lined the opening with 1X4 & 1X6 pine. I was working only from this side, leaving the drywall on the other side intact until I finished boxing out the tunnel. That way I didn't cut out the wrong location on the other side. It sure made building the lining a pain at times, though!
And here's the pass-thru completed. I managed to finish without having to take out any wall studs, which in this house are 2X6's. I did remove a couple pieces of blocking between studs - one on the near side, and one on the far side.
And speaking of the far side, here's the pass-thru from the other room. I removed the drywall in one chunk, so if I ever need to seal the pass-thru I can simply frame the opening and install the removed pieces from both sides.
We hosted the NMRA division annual business meeting here yesterday, so I had to clean up the layout and prepare for the visit. Folks were very gracious, and seemed to like what I've accomplished so far (small amount that it is).