I had to do this for awhile until i could find a more suitable home for an urchin that had become a problem due to size. I just took a piece of nori, used my long tongs and put it on the urchin as close to his base as possible. He would eventually grab it with his "feet", then proceed to devour it. He also really liked to eat celery, stalk, leaves and all the same way. not sure if that is terribly nutritional, but he ate it so what can I say, lol. Have to warn you though, they can be rather messy eaters if you do this.
