I managed to finish rouging in the second tunnel portal and contoured the surrounding landscape.

I stained the wood portals with a mixture of burnt-umber oil paint / varsol, and then lightly over-sprayed everything with black spray paint. (I made sure I stained the wood before I started gluing the surrounding scenery as the glue would resist the stain later.) I applied a thick coat of white glue around the portals an added some loosely wadded balls of paper towel to help flesh-out the hills, and then applied a smooth layer of paper towels and white glue to refine the contours.

Finally, I smoothed out some of the surrounding scenery with drywall mud. I have come to prefer drywall compound over regular plaster because of its working time. Granted, it shrinks and is not as hard as plaster but as I’m not relying on the mud for structural stability, I’m not too worried. It’s also easier to sand, and cleans up very easily.