So, now I've got the hole made. Cat loves it, btw.
The first problem I've run into is I've got 13 inches from front of wall to closest point on heating duct vent. The speaker cabinet would extend into the wall almost 15 inches. I knew it would have to stick out some, but this is the first time I realized it was going to be about 2 1/2 inches. I don't want it against the heat duct. Need about 1/2 inch clearance or so.
Next step is to reinforce the upper and lower part of hole with some 2x2 on the inside of the wall to protect the drywall and build a shelf.
After the speaker is mounted, I need to find some filler wood that will fill the 2 1/2 inch gap on the outside.
The most I could move the speaker up from where it is, is about 1/2 inch, because that's where the next floor up starts with some support lumber.