John 21:18 only says Peter will be a prisoner when he dies. He won't be a free man.
Anyhow, I don't think it matters by which design of wood he died nailed to. It's too irrelevant to become the focus of his death. Kinda like people searching for a cup. (Holy Grail. Where does it say it's even his cup? Likely one of the dishes of the guy who's house they ate in.) No one is looking for the forks and knives they ate with or the plate the loaf was on.