There is a material in RealityKit specifically for occlusion, it is called OcclusionMaterial.
Please refer to this WWDC video, which shows how to use the occlusion material: https://developer.apple.com/videos/play/wwdc2019/605/https://developer.apple.com/videos/play/wwdc2019/605/
Thanks @gchiste for the answeer..
However, I wonder why can't this Occlusion Material be available inside Reality Composer? If it is as USD, what's the easiest way to have a Box that I can reuse as USDZ that has Occlusion Material?
I will try the RealityKit method, but now I have to be inside XCode.
Wouldn't it be simpler if USDZ has Occlusion Material and Reality Composer also allows user to play with this?