Lowes has these heavy duty d-ring style hangers (in the picture hanging isle of the hardware) that you could make a slot in the top then epoxy and add two small nails to hang it from. That way it could hang below the arm of the post and be removable.
What I did on a sign was take my biscuit cutter and made the slots marked my holes for the two holes in the hanger, drilled the holes, inserted the hanger, then the nails cut off and counter sunk and finally poured the epoxy into the slot to fill the gaps and to help anchor the hanger.