After I had the subframe back on the main frame, the floor went onto the subframe:
Since I'm not a professional welder, I expected some warping in the inner frame. To still build the cabin square, I positioned the front wall and one side panel on each side and glued them together. This gave me a square reference point for aligning the frame.
When cutting the panels to sizeThe frame had less warping than expectedThe individual page elements I then to the frame and then glued them at the tongue and groove contact points. The finished cabin with the aluminum angles glued on.
Here's a video I filmed in addition to the pictures:
