Finally completed, thanks to some walnut stained pieces of pine which I ripped to size. I may replace these with some sunbleached stained wood instead, to look a little more convincing.
The liverying was done using my wife's Cricut and some white adhesive vinyl sheet. It was a bit of a chore to apply but the end result was worth it. On the next wagon I may try laser-cutting the lettering on the sheet, as I have a 3D printer becoming spare, which could be usefully converted.