Admittedly mine's 3.1/2"G, so I had to do my own thing, but I went for a brass fabrication with cast iron liners. Fabricating from steel with CI liners would be another option. Much cheaper to fabricate!
Provided you ensure it's well lubricated after a run, CI is the way to go – especially as you can then use CI piston rings in both the main cylinders and the piston valves.