I built a few boilers and used something called phoscopper rod. Used 8 inch copper pipe and plate for the ends concaved in. It static pressure tested to 300 pounds and the sides bulged a little but not at the joints so it held. It took a lot of heat using bricks and a gas burner to create enough...
There is a chemical that attacks hard steel and not aluminum, I can't remember what it's called but I'm sure a little research can be done. It's a simple chemical you could buy at a hobby shop or big toy store.
In my days as a M?C mechanic the top inside edge of the rings had a bevel that let the compression leak around them a little and push the rings outward to seal in the bore.
Anyone use a reamer to enlarge the spindle borehole? I have a 9 inch South Bend that is 3/4 inch. I wonder how much bigger I could go if it's even possible?