Work is progressing well - if fairly slowly - with the signal box. I have now added the outer 'shell' to 3 sides. The way I've done this is to use moulded brick styrene sheet for the lower half and 20 thou styrene sheet scribed to represent the planking of the upper section.
The tricky bit is making sure the two layers fit exactly, around the edges and door and window apertures. There's not really been a 'method' to this - just a lot of careful measuring, making things slightly larger than required and then filing painstakingly once the two layers are glued together so that everything is flush and no joins are visible. Slow but satisfying work!