At the back of a tank loco is space for coal. On the C15, it's a gaping hole that could swallow a couple of full-sized lumps of the stuff.
I'm not depleting my supplies of real Welsh Steam Coal (honestly, it's what I use, get in touch if you'd be interested in buying some) that much, so I filled the hole with foam rubber filched from some packing suitably chopped about to look like the coal is sloping toward the bunker whole. Somewhere I have some dense dark grey but couldn't find it so this time it was a medium density powder blue. Never mind, a coat of Humbrol black acrylic sorted this out although it looks a lot like "liquid lead" in the photo, the filling weighs nothing.
This was topped with broken up coal stuck in place with lashings of watered down PVA. A 50/50 mix of water and glue plus a drop of washing up liquid to reduce the surface tension. The water came from the pot I cleaned the acrylic brush in so it was nicely black. I'm hoping this adds extra shadows or something. Well, it seemed like a good idea at the time.
As well as saving coal, this has the added advantage of not adding a load of unwanted weight at the back of the engine. I'm nearly done and don't want to be re-balancing a model see-sawing on it's drivers.