It is hard to tell from your question exactly what you are looking for, but I assume this can be an abstract or "artistic" prop.
Personally, when I think of Tom Robinson, I think of a couple things (physically): The first is his withered/crippled left hand which was the result of an accident with a cotton gin when he was a child. The book describes his left arm as a full "twelve inches" shorter than his right arm. I'm not sure of an object you could use for this, but anything that showed his one bad arm would tap in to a key element of his innocence that is ignored by the jury.
The other physical picture that I associate with Tom Robinson is that of an axe. He is asked by Mayella to come in and "bust up a chiffarobe." Though he only has one working hand and arm, he testifies that she had asked him on several occasions to come in to the yard and do other little odd jobs for her like chopping wood. Link Deas interrupts the court to announce how good of a worker Tom Robinson is. I think an axe could symbolize both his strength and his ability/willingness to work.