Putting a polygon at the location of a clipping polygon to fill the void

CesiumJS
1

I used clipping polygon to hide part of the google 3d Tileset base map. I then render my own 3d tiles at the clipped region. When I zoom out far enough and my 3d tiles unrender, I am left with a big void on the globe. I want to fill the void with another polygon with the same position/coordinate as the clipping polygon so that the void is not so obvious. (here is a sandcastle example of clipping Cesium Sandcastle)

When I tried to do this with
const fillVoid = viewer.entities.add({
name: “void”,
polygon: {
hierarchy: Cesium.Cartesian3.fromDegreesArray(coords.flat(2),
material: Cesium.Color.GREEN,
},
});
My own 3d tile is now all green.

Is there any easy way of doing this?