How to cover a hole in a model cut by a mask? (Shader Graph)

I've created a shader in the Shader Graph that cuts out part of the walls based on a mask. I'm trying to come up with an idea on how to cover the tops of the shortened walls inside the shader so the inside of them is not visible. Haven't found any answers on Google unfortunately. I'm thinking that somehow I'd have to add new vertices at the height where the mask cuts off the rest of the model?