I have done exactly this for my “max detail” 3mm set up. Let me pass along some lessons learned.
For flexible base material, use an appropriately painted cloth.
Nails are areal bitch. My mat is canvas, overlaid with silicon caulking and flocking. Nails are hard to push into it, distort it, and make holes.
My solution? Those long, sharp, sewing ouch pins with a ball on top.
I make a bunch of trees out of those and use them to anchor the margins of the woods area, then I throw easily removed scatter terrain into the middle of the area. It can be easily removed when troops enter.
We get slapped around, but we have a good time!