Heres a idea of how these are made. My first set of Rigs I'm happy with. They are two pieces to cut down on costs and time. People in my area can't buy 200$ rigs. So I used a grommet. You never pull the swing out so I figure it doesn't matter and I don't have to seal a downstem or a fitting on them. Plus they are easier to clean.
mouthpiece shaped
base (can) shaped
bridged and welded together
from there you add feet and mibs.
Here is the complete rig (just missing the grommet)