The lid is back off...

Actuall, here the roof is sitting on the back of the bed rails, Where I am trimming the outside wall level with the bed rails.

Then I clamped a 1" x 1" tin walled sqare tube between the outer and inner walls. This is a shot from below.

Another shot from below.

Now the roof is off and upside-down and the inner wall is trimmed. By welding the outer and inner to this square tube, the rear wall will gain stiffness and the thickness will form a sealable surface on top of the front bed wall, yet to be built.

Maybe a better shot.