How to hide geometry behind floor/walls/ceiling in mixed reality/passthrough?

I've tried using Scene with my room scan but objects below the floor (20m) are still visible. 

If you're wondering "why" - I'm using a portal effect, and I want to "hide" the portal VR environment outside of mixed reality. I feel it should be obvious, but I can't figure it out, so thanks a lot in advance for any help!