I have an old set of rules, Striker, which uses two densities of trees and two of undergrowth. I also looked at possibilities of showing light woods and dense woods. I came up with a few trees glued to tongue depressors for light woods and a solid layer of foamboard (black) topped with lichen and/or foam flocking and mounted on 1″ nails for dense areas. A local guy does similar and I happily stole his idea. I use common nails which have a reasonably large flat head. Glue them head side to the bottom of the foamboard and cover the top as described above. Black works better than white. I have also seen where a guy made up a base to put the dense woods onto which is the size of the foamboard wood shape. The base (his was solid) made out of the felt which can define the density of the undergrowth, use your two colours or in my case I have a green felt with a lot of white specks (sort of flattened tufts) for light undergrowth and a solid darker green for dense.
Cheaper than lots of trees for the dense and looks great. I also play almost all 6mm.