I use 1/32" X 1/2" Strap. Just 1028 hot rolled. You get it at any steel seller.

To make the concave form just take a piece of 3/4" round stock and use it as a press, and hammer the strap into a round mill cut in a 6" long piece of steel. Make the cut with a 3/4" or 7/8" ball end mill. The rib will not look perfect, but you need to fit it anyway, so it's tight to the barrels. When it's soldered up nicely at the edges, take a large rat-tail file to smooth out the bumps, and then a piece of 120 grit Wet-or-Dry over a 3/4" dowel, to make it all pretty.
It's a lot of hand work, but not really very hard to do. If you just take your time and it comes out beautiful.

Steve