Not sure of the size, but 10mm ACM should do the trick. No need for a frame, unless you wanna get into a lotta extra costs. Use 2 really good sign brackets and maybe find some you can use two bolts and nuts for each one to make sure there's no play in movement when swinging around up there. You could also attach a small chain to the main pole and the bottom area of your sign to help prevent extensive blowing around.
I would not consider using .080 or .125 as that would be too heavy for such an application at those sizes and only hanging at the top. Too dangerous. If for any reason, that would let go, you'd either decapitate someone or give them a dent in their head that just won't go away.