Detecting backfaces to generate a cutaway with a post process material (fill gaps after opacity mask)

I’m trying to apply a post process material on the backfaces of elements who are getting cut by a opacity mask, it’s a simulation of a construction where the mask is animated from bottom to top, and the post process material would be a simple color.

Can someone help me in this?

I find this tutorial https://zoltane.com/pages/unreal/cutaway-views/ but cant understand how to make it work

There’s this post who archieve something like I want but he doesn’t answer how Applying material to backfaces - Rendering - Unreal Engine Forums