With the carriage body sides joined together and a floor keeping the lower half in shape the obvious thing to so next is make something to so the same job at the top.
The way I build our carriages is generally to have a fixed roof and a removable floor, and within that is a false ceiling to give support to the fragile top of of the body.
What you see here is a three layer laminate.
The bottom layer is a neat fit inside the top of the body. The middle one sits on top and at this stage is slightly over-sized.
Both of these are necessary parts of the structure of my roofs but the top layer is an additional one I've added this time to give it more rigidity and prevent it flexing when the roof skin is stretched over the top later on.