Back when I first set up the walls of the bus I made a card pattern of the floor space which I traced on styrene to start things off. I saved that card pattern and used it again (slightly modified) to cut out a piece of 1/8 inch sheet wood that fits between the walls as they stand now. That wood piece was then glued to an oversize wood block which will form the roof and the thin sheet part will make it possible to always fit the roof to the model in the same way because it fits within the walls. I then set it on the model, turned it up side down, and traced the bus walls onto the wood. That was then cut away on the band-saw.