What a great idea for a thread! Anything that requires a bit of imagination and modelling, but not too much rivet counting or lining transfers definitely gets my vote!
Anyway, if you look in Colin Marsden's book "Rolling Stock Recognition 3: Departmental Stock" you'll find all sorts of fun things that BR knocked up from otherwise obsolete coaches and wagons.
Below is a 'Palbrick' match wagon used, I believe, to connect container wagons with their own type of coupling to shunters that didn't have that type of coupler. I photographed one at some point in the mid 1980s at Old Oak Common, and a couple of years back ran one up using a whitemetal bolster wagon and a few offcuts of plastic strip.
Cheers, NeMo

