I'd say go for one of those long sheets of pink insulation foam like the one pictured below:
You can get them from Lowes or Home Depot.
Then start carving away at it. And you don't even have to sand it or smooth it too much because the rough foam texture will look like the 'woody' texture of his staff already. Just be careful when carving it so you don't go too thin and have it break. Then you can reinforce it with some mod podge or matte gesso so it'll survive a few rounds of convention wandering.
^^ I just have to say how much I enjoyed that movie, btw. Saw it in theaters, then twice on dvd. I might have to restrain myself from squealing like a little girl too much if I see any RotG cosplays at NYCC this year. ^^;