Is there a plugin to make a geometry completely solid?

[Gaieus]

Hi Jeff,

Though this tool will not make things "solid" in the way it is used with the "Solid tools", if you only need a "cap" for your cut model for viewing purposes, use this tool:
http://sketchucation.com/forums/viewtopic.php?p=172583#p172583

[Dave R]

Since SketchUp is a surface modeler, you'll never have truly solid "objects". Until you slice through the objects with Zorro, there's nothing defining where faces should be placed to close up the "objects".

You'll have to close them up after slicing.

[Box]

And intersecting a face through the object will at least give you a closed face for one half if done in the correct context.