Is there a current source for carriage chassis trusses? I used to obtain etched brass trusses for my LNER scratch-builds from Etched Pixels, but they are suddenly not available.
Have you considered eBay 'spares or repairs'? You can often find job lots of carriages with missing bogies or roofs or some other damage. Cut off the trusses you need and keep anything else useable for future projects.
Just a thought.
Cheers,
Chris