The inside ceiling is installed after the ribs. I trimmed the 5mm ply to fit between the walls. The ribs had a good ribbon of PL glue run across each one. I laid on my back and forced the ceiling into place with my feet. When it was tight against the rib, a one inch brad was shot into place with an electric pin/brad gun. This is enough to hold the thin ply until the glue sets up.
On the top of the ceiling small blocks cut from scrap 1x2 ribs were glued to the ceiling and wall for reinforcement. The small blocks went around the curve adding much strength.