"D" Minus 13...

Take a a couple of sheets of newspaper torn into strips, soak them overnight, and in the morning boil them into a mash after adding a pinch of salt -yum.  (serious about the salt- it is to get rid of any bacteria that could cause mold).

Add some PVA and wall filler and voila- goop. Bon appetit.

The next step was spreading the gunk liberally around the base of the trench pieces and the rocky outcrops, and to smooth out any joins. 

And here it is- looking as if someone has thrown up their porridge over it.  Maybe a Scottish rugby fan after watching France hand them their butts in the Six Nations tournament (Gloat? Moi?!?).

This is a large but relatively thin model which will be prone to flexing.  There is no way to completely avoid some kind of warping on a piece this size, and in this climate (hot, extremely humid summers).  So it seems more realistic to aim at reducing warping rather than worrying about trying to eliminate it altogether.

The white patches are pieces of an old dish cloth soaked in PVA and added to those joins that might be vulnerable to cracking over time as the trenches get moved about a lot.  The PVA-soaked towel will give and stretch a bit as it flexes, rather than just cracking under the strain as would a more brittle putty.    

I learned through bitter experience that one way to reduce warping is to carve out as much of the main contours as possible directly out of foam board. The papier maché is then used only just to texture the surface, rather than being applied in any great depth. In the photo most of the grey gunk you see is over the plastic trench itself or on carved foam. 

Where is comes into contact with the base it is only a few millimetres thick- and it is already drying out nicely. But at some point I need to add glue--soaked towelling to the base, and warping well may be an issue.  When I do, I'll weight down the model again to see if that helps to keep warping to a minimum.