Material "Cut" effect, how to "Fill" the object.

I have a simple “Cut” shader effect.
But the object looks hollow.

What I want is pretty much to make it look like it’s full.

This effect will be used in a Top Down game when the player goes inside a building and it will be “Cut”, so the player can see.

I don’t want to use a texture, just a solid colour is fine with me.

Here is the clipping plane material. It is not final but I want to get it working on this simple “Cut” shader before implementing it on my final material.