Wool would work. Might by tricky to sew it in though. How about slightly too big clothes, like you suggested, glue some yellow wool with fabric glue, so it sticks out like straw, then tie round wrists and ankles with string. Trim a hat in the same way - could be any old hat, I suppose. Rafia might work, but might be itchy. Sorry. That wasn't all that helpful. Love the idea though! My two have got cute witch outifts from Asda - they had lots of nice ones, but I wasn't looking at the little boy costumes, I'm afraid.