Paint the Concertina and assemble with the spring over the End Plate spigot. Pass the spigot through the hole in the concertina and fix it with the fixing washer. Adjust the attitude of fixing washer so the End Plate is equally spaced 1mm out from the concertina and then secure with a tiny amount of super glue.
Coupled with left over Keen Systems flat plate couplers from the Close Coupling System.
Coupled with the Hornby NEM couplers.
The carriages will go round Hornby radius 2 curves.
With care this can be done without taking the carriage appart.