This is not meant to discourage you, but I imagine that there aren't any because wood is easier to work with and requires less skill. It's easy to cut, assemble, etc. If you have the metal working skills then go for it! I would take a look at as many designs/plans as you can and then adapt them to metal. If you decide to go for it, you should do a progression thread on the construction. I'd really like to follow it!

Good luck!