Going to do a little redesign and rebuild while I'm not too far in.
1) Only use 1 2x4 for the lip of the frame instead of 2. The second isn't providing much in the way of extra strength and takes away an extra 3" of viewing window in both directions.
2) The old 2x6 lumber around the outside of the viewing window has had me antsy from the get go, so it'll be replaced with new lumber to put me at ease. The old lumber is very hard, heavy, and seemingly brittle to a degree.
3) Between every wood on wood surface, I'm going to put Liquid Nails Heavy Duty Construction Adhesive. It'll add about $60 or so to the build, but well worth it for the extra insurance.
This allows me to rework a little bit of the structure and change some of the assembly order.