Fredo6 on the Sketchucation Forum has created a new terrain maker called TopoShaper. It will take your grouped contours and create a terrain surface from them. This is not duplicate of the Sandbox tools, but rather a significant upgrade in power tools for landscape architects, landscape designers, and site designers.
Here are some of the ways it is different from Sandbox tools: It will fix breaks in your contours (say from elevation labels) and allow you to manually make changes too. It let’s you view the terrain and contours in multiple views so you can see and edit as needed. The terrain surface sits close to the contours but does not use the contour vertices. So it doesn’t pull to a point, but rather fits the best concave shape it can for the whole length of the contour. It also allows you to set the size of the terrain grid so you can choose the level of detail that you want. The terrain that it makes is a grid or quads rather than a network of triangles (a TIN). This will let you edit the surface with Artisan or other plugins much easier. It also creates a skirt around the edge of the topo for a clean look.
He has a manual to show how to use the series of tools/buttons. And the plugin can be downloaded here: http://sketchucation.com/forums/viewtopic.php
Note that you need to have the latest version of the Fredo Library, LibFredo6 v4.8a, in your plugin library for this plugin to work. PLEASE consider donating to him via PayPal if you think this or all of the other plugins he has made are valuable to your SketchUp process.