thats not the back... thats the dome you could use the ball for... but i dont recommend it, as Fett's helm basically has the form of a skull, an oval more then a round ball...
and well if you're worried about how to curve the back... have you tackled the mandibles yet... or the T-visor.
plexi-glas , fiberglas, ABS, sintra... all materials that can be used for a bucket... even aluminium can be used for one.
the plastics can be bend with an oven or a heat gun, or you can try vac forming...