Add Terrain Skirt in SketchUp

[Geo]

Why fiddle with a plugin for such a simple task?

Creating a 3d STL terrain model from usgs or google earth for CNC

The model must be a “Solid Group” or “Solid Component” and then exported to STL I. e. A closed volume. Intersect a cube with the terrain and erase what you don’t need. Here’s an illustration. Download the model and have a look.