With a jigsaw, cut the hole for the round electrical box in the second 1x8 and nail it to the post assembly. Trace and cut the hole for the outlet box in the second 1x6. Tack the 1x6 to the sides ...

these three boxes cover about 99 percent of your needs. The purpose of electrical boxes Wiring connections—where wires join an outlet, switch or other wires—must be inside an electrical box.