Why start with a 1/2" scale flatcar that may or may not be available…you probably are modeling in 1:29, so you have two ways to go.
One way is to grab a USTrains or Aristo flat, and build using styrene sheet, and shapes on it.
The second way is to try and find an old USTRains, outside braced box car and bash it with styrene sheet and shapes. You should look for trucks other than archbars for this project, although you don’t want roller bearing trucks either; and if starting with a flatcar; look for ones with steel underframes, not trussrods.
…of course if you are into Narrow (Gauge) Mindedness…!!!