While waiting for delivery of some plastikard I've ordered for the build, I made a neat'ish drawing of the pieces for the front tank...
I wanted to use the same tank construction for the rear tank as well, so I sketched out a scheme for the rear tank (and coal store) to fit inside the back of the cab as per the real engine,
I spent a good half hour 'carefully' removing the bottom part of the back of the cab...
using my mock-up cardboard tank I fixed a profile of the coal bunker to it (slightly narrower than the tank); fits just right into the rear of the cab...
going round a curve the loco will pivot; a check on the worst-case cornering position the coal bunker doesn't touch the cab side - super 😀
The coal bunker will work as an add-on to the rear tank, although I need to take a bit out of the tank edge facing the cab for the coal to sit in. The final coal bunker will be best designed once I get to the build stage.