What are you attempting to achieve? (Keep it simple and clear)
I want to hide a union part behind an invisible wall so that its invisible from the outside but visible from the inside
Are you wanting to make it seem as if a hole does not exist at all when viewed from the outside? If so, one method you could try is to place a decal on the outward-facing side of a part with max transparency.
This makes it so that the player will see the decal only from one angle- the outside. The decal could be a simple image of the surrounding texture.