Mr Johnster, You can source 3D printed Fishbellies from shapeways.
Somewhere in the GWR prototype etc. Section there is a thread I did several years ago regarding building 3 (Yup, 3) K's autotrailers.
I stalled when I got to the glazing of them as I wanted to try and make my own "semi flushglazing" as the whitemetal sides are way too thick.
They are buried in a box as other things peeked my interest....but one day...
Anyway I'll try and root out some pic!