As others have said, don't use the co2 for the shielding gas.
One thing you can do is to make a dimpling tool that I posted a while back. There are some great recreations of it in the soldering ss thread in the sticky section. Basically what it will do for you is....make the weld in mig much raiser. Your only real worries will be your starts and stops leaking. Its a bit more difficult tieing in with the mig process. You may need to go back and fill in a pin hole or two but that should be easy enough given the usage of the end product here.
I would post up pics of my tool but im on my phone right now. Sorry.