How tall is the Union that you have there?
Basically negating is taking a percentage out of the solid Part or Union that is there. If you take a large section from a small Part then it will likely be visually and physically the same size.
If you have a large Part like a thick wall or round turret, and you remove a tiny doorway through it, it isn’t likely to be physically the same size as the hole that is created visually. You can make it better by cutting the large cylinder into smaller sections (so the percentage is better), or by creating a larger hole and filling it with door frame Parts.
Another way is to make the Union CanCollide off and put transparent walls where the Union is.