I've been working on the Mikes Models whitemetal kit for a Midland yard crane for Callow Lane recently.
A real example was used in the small goods yard at Nailsworth and is well-illustrated in
'Gloucester Midland Lines, Part 2: South' by Neil Parkhouse (Lightmoor Press).
Although most of the components are whitemetal, the main body of the jib had to be fashioned from a square section piece of balsa wood, on which the four corners are sanded flat to make an octagonal-shaped structure.
I did take some photos during the construction of the main mechanism, but seem to have lost them in the process of transferring them from my phone to my desktop PC (they are there somewhere, but I just can't find them at the moment!).
So here are some photos of the completed crane, prior to painting (the wooden jib was painted, sanded and then the gaps in the wood grain filled).
In situ on the layout. The crane will be primed and then sprayed black or dark grey and then weathered. The (colour) photos in the Lightmoor book show the yard in it's last days and the area around the base of the crane are weedy and rather unkempt, which is the look I shall aim for on Callow Lane.