Hi CJ, I can make threads sticky, perhaps this is a good one for that. It's a good suggestion, thank you.
I have made many cast cores with windows at this point. I played with quite a few methods, and as always the simplest solution has turned out the best for me. I cast the core with an additional piece to make the window void stuck on the side of the mold. Best location in my opinion is a spot that shows the bottom of the fuel, barely, and then has a view into the burn tunnel, on the side of the core.
The window is just captured against the core with the outer cob layer, with some fiberglass rope as an expansion joint around the edge of the glass on both sides. This has worked very well for me. If you need more detail, we could start a new thread and I'd be happy to elaborate on the process a bit for you.
P.S. I'm close to solving the muck issue!