I'm not even sure your problem will be solved with a Spatial. But it's worth trying. Make the Spatial the parent of the object, with a child of a MeshInstance (for your cube model), then an Area (also child of Spatial) and a CollisionShape as a child of the Area.
But you could also skip this entirely and handle the movement yourself just put setting the "translation" variable directly. If the cubes don't collide with anything, then don't need the area or collision shape. Just check if they are off the screen to remove/replace them. Depending on how your game is setup, this can be simple. Like just checking if the "translation.x" is greater than some number.
If the cubes are just used as a background (for example a star field) it would be better to just have them teleport when exiting the screen (moving from one side to the other edge) rather than constantly creating and destroying objects (this can cause slow performance).
