nav_edit - Is there a way to 'easily' create areas on complex displacements?

The automatic NAV generation keeps creating a bunch of areas on flat and/or noisy displacements when that's not always needed, and keeps "confusing" TF Bosses while they walk around these spots. I created areas on ever flat surface from scratch to prevent that from happening and decrease the file's size, but I'm currently struggling to do the same on displacements that are wildly geometry-painted. I tried using the cordon tool and creating solid blocks replacing the displacements on a "test" map to later use this same nav on the original map, but it was even more complex. I currently have in mind merging these blocks, but before trying it, I'd like to know if there's something else I can do. Thanks!