Please clarify what you want to do with this structure, as it is not really clear from your post.
Having looked at your structure, I can say that you should increase the lattice parameters in A and B directions by 10-15 Angstrom. After that you may create a two-probe device from bulk, using Device Tools, if this is the actual purpose of making this structure.
If you want to create a structure periodic in the C-direction, you may mirror your present structure, using the Mirror plugin in the Builder, just tick Fractional and Copy, set N and P to 0 0 1, and then click on Apply. After that you have to double the lattice parameter in the C-direction in the Bulk Tools -> Lattice parameters. Finally, you get the structure as shown in the png/py-file enclosed.