Did you get your costume made? My DS needs one too, and someone I know on another board posted this suggestion that her dd's preschool made every year:
Get a plain white t-shirt (a little big), soak it in coffee for a few hours until it turns tan/brown, then get fabric paint and put a design around the top collar (zig zag all the way around, dots in the spaces, etc), take scissors and cut up the bottom and sleeves 3-4 inches so it's in strips.
I thought that sounded easy too. I bought the t-shirt yesterday, and some tan Rit dye instead of coffee (we don't drink coffee, the rest would be wasted) and I"m going to try it this afternoon.