Further progress.
These next few photos should whet your appetite if you faintly desire to do something similar.
But let me tell you from the outset, it is quite time consuming, and fiddly.
And honesty bids me tell you that there have been some re-runs, un-gluing, re-packing, re-gluing, scraping, measuring and re-measuring.
So to recapitulate.
1. Choose one wagon on which to completely remove the sides (except for the brake end and the under-roof frame and ends). This becomes the BASIS for everything.
2. The other three wagons are used for more replacements by cutting and rejoining sections and NOT the complete sides.
I will describe how the cutting and rejoining can be done. I won't say should be done because there are probably other ways just as good.
Here is a complete new side set up on the chassis and with the door remaining to be done.
(See post #11 for plans and prototype photo)
Notice the glass sheet - it was used both for gluing the parts together and to keep the body aligned under weight while the glue dried

Here we have the BASIS body shell, set up as one complete unit with under-roof frame and body ends all intact and as moulded by ROCO.
Both sides of the BASIS will be set up this way leaving the door to be manufactured from other planked side material.
The beautiful thing about this is the the width of the door is not a critical measurement.
It will be the space as shown here.
Whether that is the prototype width is a detail with which I shall not concern myself.
You might notice a misalignment at the top of the door space.
This will be aligned at final gluing with the door in place.
Now we can get to some detail about how these sections were cut.
Remember this new side took material removed from BASIS, and material from ONE other wagon.
Both complete sides were already removed from the BASIS wagon, but the cuts are the same (or similar).
Here is some detail from both sides of that ONE wagon as cut.

SIDE ONE - The sense is the "under-roof frame" is a word - do not read it as though I am saying from under the roof frame.
The red lines are cuts. I used a scalpel/hobby knife following the natural barrier of the strapping as modelled by ROCO.
So a ruler was not required.
A final cut on the inside as exact as you can makes the snap-off fairly easy.
On the right hand section you can see how I cut around part of the door frame.
This is important because it supplies the detail of the frame and door hinges etc. for the right side of the door.
The left hand section becomes the adjacent right hand piece to the right hand section.

SIDE TWO -
On the LEFT hand section, like Side One I cut around part of the door frame.
That supplies detail of the frame and door hinges etc. this time for the left side of the door.
The right hand section need not be cut out because you already own one from the cuts on SIDE 1.
(Because you now have the frame and hinges for the door, the same will not be required from the BASIS sides).
I can probably slice off one or both of the destination/loading label holders?
And I can probably re-glue one in a better place if they slice off cleanly using a chisel blade or a wood chisel.
I guess there should be only one on each side.
And I will remove the excess door-bumpers for the opened doors.
That will suffice for now.
I expect I will not do much more for a week or so because other chores beckon.
Thank you.
Kimball