Look for local CNC companies. The cheapest method is if you have access to CAD, you can design them and have the CNC computer mill them. If you can design some common stock, it would be cheaper then milling from a solid chuck of metal.
A metal port sounds conter productive to me. Wouldn't the metal ring like a bell and become too good of a sound/vibration conductor? If it becomes a problem, you could line it with a rubber like compound or glue a thin layer of foam to hidden surfaces...
Yes, the metal would ring like a bell. Even heavy pipes. But I like the sand casting idea because one could make it as thick as you like.
Don't forget the possibility of using fired clay. Highly malleable and has resonance properties more like concrete. Although I might consider mixing in a bit of powdered pvc or plastic to make it a bit less vulnerable to impact cracking.