Thanks for your reply!
In my object, the bottom plane is a “polygon”. I have add all the other facets except the bottom plane. But G4TessellatedSolid requires a fully enclosed space.
I try to triangulate the bottom plane and then using triangular facets. And this is the result:
But I wonder if there is any other elegant way to do this? (the bottom is actually a plane, the point inside the bottom plane can be removed)