I made a bunch of trenches in 12,6 and 3 inch lengths using thin MDF bases with a layer of filler on them, and the trenches scraped out using a piece wood. Painted the whole lot mud coloured, did the trench depressions a darker brown and flocked the upper sides. Job done. I use them with 2mm and 6mm stuff.
They can be arranged in various configurations.
"Mistakes in the initial deployment cannot be rectified" - Helmuth von Moltke