Thinking about the potential overheating issue, I reached out to a former neighbor who is a ceramics engineer. He's an expert at heat management and literally a rocket scientist - helluva nice guy too. I explain my issue and sent him some pictures. He agreed that ducting outside air into the space under the mantle should be enough to stop overheating, especially in the winter when daytime temps hover in the 20s and nighttime can get sub-zero. Adding a vent opening at the top of the external popout would help as well.

I determined I had room for some 3" ducting so I got to cutting...