Allow sphere in colliders only

I'm trying to figure out a way to only allow a sphere(s) to appear in the series of joined box colliders see Screenshot, I've set up.

I've written some test code to see if I hit a collider using the Physics.Raycast with a layer mask, But I cannot figure out how to make sure the Physics.OverlapSphere would check to see if it's ONLY within the bounds of it.

One idea was I could do a series of colliders that circle the other colliders and if the Physics.OverlapSphere hit one of these I know it's not within the bounds. This is only for the editor part as when a user runs it the Sphere's/Trees will already be created.

Cheers.