For a quad hopper there are 8 doors. This photo shows the parts that make up two such doors. The longer pieces are the permanently-fixed sheets, and the smaller pieces are the swinging doors (in the real world, that is). Each piece has a slight slope on one side. The longer of the short edges is 3'4" and the shorter is 3' even. The dimensions of the pieces were made to fit the model and a bunch of trial-and-error work.
To make sure that all the doors have about the same vertical length, I made a short stick with a mark on it that I lined up with a distance mark on the side of the car. This stick was then clamped to the side and then the long piece of the door was glued in place by eye-balling it to line it up with the bottom of the stick. It was the best idea I could come up with at the time. I'll have to think these parts through again, because I am not happy with how it all came out. Things just weren't lining up correctly. Maybe the prototype drawings will give me a clue (once I receive those).
This is what the car looks like after all 8 bays have been installed.
Next are the sheets that make up the sides of the doors. I did the inside sheets first. Through trial-and-error I found the right size for each. As you can see, the interior is starting to look kind of shoddy. However, my plan is to install a coal load into this car, so the interior will not be visible when the model is finished.
The last step with regard to the doors is to install the outside side panels of the doors. Note that these are at an angle. Again, much trial-and-error to get each one to fit. Some gaps are visible. I will see about hiding those later on.