Day 1: The bunkers were built out of foam core, pinned, glued and sprayed black, with successive, lighter coats of grey.
Foam core was used for basing (mistake... it eventually warped after it got soaked with the scenic cement. Use hardboard!) and some hills were cut out of polystyrene.

The main board was based on hardboard. 2" polystyrene was glued and cut with a hot-knife to create the cliffs. (note: The hole for the gun emplacement was cut out before gluing.)
The gun platform was cut from 3/4" foamcore, the edges were taped and pins were inserted to make the "rivets"
The gun itself is a piece of PVC pipe with some "lock washers" glued on the end to make the muzzle.
The bunkered walls of the emplacement were made using some extra parts from a Necron Monolith, and the monolith's "crystal holder" is actually supporting the gun barrel (with some help from a piece of copper pipe).

The whole thing was coated with "FoamCoat" and then sprayed (heavily!) black.

The wall was then secured to the bottom with some right angle clamps and polystyrene rocks were added.

Day 2: Added some foamcore walls and coated the ground with scenic cement to hold the turf.
(When this dried, it caused the paper on the foamcore to shrink a little, which caused it to warp, slightly. Easily fixed by trimming the worst warps off and then gently bending the remaining foamcore back into shape, but annoying...)

The cliffs and rocks were sprayed with a dark grey, light grey and in random spots, a VERY light coat of brown. The gun emplacement was hit from a distance with some silver, to give it a metallic sheen.
After that, some dry-brushing work with a large (2") brush gave successively lighter shades to the rocks and cliff face. Random spots had ochre and brown touches added.

A "header" made form foamcore was added to the gun emplacement. Although it was not originally planned, I felt it needed a more closed in look.

The final touches were done and the piece was finished.

(Photos to be loaded soon)

Total work time: 3 hours (although there was a lot of time spent waiting for things to dry)