This is a bit of an instructional post so for some readers it may be granny and eggs but if not I hope that it helps at some point.
In my experiences with plastic wagon kits from such as Slaters and Parkside (Peco) the way that they do the corners sometimes leaves a little to be desired (or more realistically fettled). The designer attempts to make not only the corner timbers marry up but also the corner strapping where fitted too. When making my own design of wagons via the Silhouette cutter I always make the iron work as 10 thou overlays.
I digress, having got as far as fitting the first of the outer Veranda partitions at first try there was a gap of around half a mil or more.
That shown above is the other end which is still to fettle. This is the end that I have done and it does fitt cleanly when pushed into place it was just difficult to get it perfectly in place and take a photo of it.
To achieve this snug fit I had to file scrape and otherwise nibble away at the two bits highlighted on both edges of the partition.