Scribe/trace the same arc of the guitar body onto a small block of wood.
Cut out with jigsaw - you then have a positive and negative
Sandwich the metal you want bent in between, then into the vice and tighten.

alternatively you could place the metal onto the positive template you just made and directly hammer into shape, dont use a metal hammer

A pair of pliers either end of the metal piece, place rags in the jaws to protect your piece and bend...

Big hammer - wack!!