All blocked in!
All dropped in.
It's not super obvious from here, but you can see the previous center section on the right side of the picture? It's been leveled and supported. The new corner section is just assembled and lying on the ground. My method was to make it, then level very precariously with blocks in corners and maybe a couple midspan, then attach it to the previous section with many nails. This corner section is much lower as you can see in the bottom center of the picture. It will get raised up then attached to the frame on the right. That gives me one solid side as a reference.