"Form a slight tumblehome" say the instructions. I wasn't looking forward to this bit.
Tumblehome forming is something I've managed to avoid so far in my modelling career. By studiously not building any coaches, I've not faced trying to put a gentle but accurate bend in brass sides. A job that if you get it badly wrong, can ruin your kit.
Looking at the profile of the partitions, the shape required is that of a fat bloke wearing a tight t-shirt in profile. At the top, the side is straight but further down there is a bulge.
With no real idea if this would work, I stuffed each side in the GW Models rolling bars and introduced a gentle curve into the main panel. Fortunately, there is sufficient space in the centre doors that I can do this in two halves - the whole side is longer then the bars. Well no-one told me I'd be building O gauge coaches when I bought them did they?
Anyway, a gentle curve looks about right and they sort of match the partitions. Hopefully a bit of pressure will bring everything into line once soldering starts.