Bob - you are correct. All of the existing side extrusion stock is fluted/ rib sided. Been thinking about ways to make smooth sides (and you might have seen my Skytop 3d print of the tail end?), but no easy plan as of this time. Thought about using flat stock, but don't really want rivets or screws to show through the sides. I will find a way to make a car with this part one way or another. Having built a new drive set for my Richart Cascade, now need some of the right cars.
Jim