The construction is plasticard, with Slaters brick laminated over 20-thou cut to form window and door frames, and 40-thou black plasticard for strength. There are lots of internal walls and floors to ensure rigidity, however there has still been a little warping. I think it is controlled and minimal on these small buildings, but I can see why stronger, more stable materials such as Plywood and Foamcore board are commonly used for larger buildings in 7mm scale.
This project is further complicated by the buildings having to come apart to allow the lid to close, and provide access to the point switches and electrical connections, as well as being shaped to fit in the box (not always parallel to the sides), with openings for the trains!
Tonight I have added final details such as lead roof flashing using tin foil, and door handles from staples. I need to make a set of doors for the loco shed/smithy too. Window glass will be added after painting.