I've had some trouble with using cardboard or corrugated board as terrain base plates, as any applied wet filler, paint, glue etc will make the board curve.
But maybe I found a solution:
Wood/white glue the board down first (or after) onto a piece of flat plastic and weigh down flat. This will set/lock the paper fibers, I guess. Then the terrain can be built, then peeled off the plastic. I actually did the reverse tho--build, then glue down. Seems to stay flat but I need to experiment more.