Thanks for your input Mike. The Hereford site would have been pretty barren in the winter, I imagine. Although it's overgrown now with trees, the older pictures suggest they were kept under control but would have meant no real windbreak. The Hereford site was also lined by a stream, albeit in a deep ditch, so I imagine the whole place had a general dampness to it (an apt description of Hereford more generally, some might say!).
There was a similar canopy at Marsh Junction in Bristol, at least judging from this picture from another thread (linked below), and a much bigger one but of broadly similar construction at the Godfrey Road depot in Newport. I assume at some point all of these various trackside structures would have had some form of engineer's drawings or similar, and that those must exist somewhere in an old BR Archive - even a simple modular/ largely pre-fab structure would have needed some form of instruction book for those putting it together (I would assume). Being nearly 5,000 miles remote from the NRM at York, I doubt it's a question I'll get to research any time soon, at least beyond what is available online and in the memories of those contributing here.
I have pieced together a 3D model of the structure based on all the photos I can find, which I'm happy is sufficiently accurate for assembling a model in a much reduced scale and with my limited modelling skills. Screenshot below, in case it is of service to anyone contemplating anything similar (happy to share the model too, if useful).