@unknownuser said:
it'll never be able to make it a solid object with Sketchup's native tools..
That's not really true, any shape can be made solid using nothing more than the line drawing tool. The trick is to think of the surface divided up into triangles (or quadrilaterals where you can make four coplanar edges)...
Manual Triangulate.jpg
...you can see here how I am adding a surface to the top of the uneven curve by simply drawing in triangles one at a time. Once they are all drawn in, you can use the eraser tool (with Ctrl held down) tool to smooth those lines into a continuous surface.
The many skinning and lofting plugins available do make this a lot quicker and simpler, but doing it by hand can sometimes give more control over the triangulation so that the "undulations" of the resulting surface are as you would like - and doing a few manual ones will also give you a feel for how the "skinning" plugins works for a better understanding of the variables in their setup dialogues.
