My son was the tin man last year. We took a brown grocery sack and spray painted it silver cut holes for head and arm and drew on a red heart.
For the cap we took a large funnel and painted it silver added a piece of elastic to wear under the chin to hold it in place. We also painted his face silver with a little red on the nose.
He wore grey sweatpants but you could do tights if you wanted. It was a hit at his preschool class. Plus, it was nostalgic for me to see that kids this young actually know who the tin man is. We used a grocery sack instead of cardboard box so that it was a little more giving....he could actually sit. Enjoy.
Thanks to Misty for this great idea!