A few days ago I started building the diorama -- here is a first look at the construction.
In brief, the diorama will be constructed from pieces of foam board that are covered in simulated grass and model trees. The foam board will be stacked up to depict the ridges and ravines on the battlefield. The first step (and what I'm working on at the moment) is drawing contour lines on the foam board and cutting out the shapes.
The northwestern section of the battlefield is now almost done (one tier is remaining). Below you can see what it looks like at the moment. For perspective, I've placed on the foam board some of the British troops that fought in this area.