Well considering that it is so huge, wood would definitely not work for this because it would be kinda tiring to carry it around the con. I helped someone out with this before.
For the entire key I suggest makin it out of craft foam that you can get from the fabric store. Than cover it with plastic sheeting. Once that is done paint it. For the chains, you can out of craft foam as well and repeat the same process. Here's a site that can help you, it's amethyst-angel's website (it's mostly armor, but the principle is the same). She does a lot of great props.
Hope this helps ya