Hi MMTS,
Those wagons will be quite large and as such should be much more complete that the small stuff (OO). There ought to be copies of the external strapping inside, which acted as keeping all together without cutting into the wood. An easy method is to use black plasticard to make it, I think that 30 thou will look OK, but 'in scale' it MAY be a bit thin.but will be easier to works with.
Each side of the doors is the most difficult as the knees were tapered from the bottom to the top, with the intention that the wagon wouil;d be assisted to stay straigh - it failed in that as loads of coal, or minerals, dumped in them over time made the top edges of wagons belly outwards!
If you run a new knife blade down to outer edges of the strapping that will provide a 'stop' for the black paint and could help to give you straight lines.
They look very bare without it.
However they look good and Thanks for the photos. Are the transfers waterslide or theose tererrible rub down variety? I cheat with those and put them onto clear transfer film, and then treat them as waterslides.