Doing one-off exhausts, we used copper wire (doesn't have to be copper, of course) to establish shape, then took to exhaust shop, and had them bend to the wire. The corners may be too big to do this on, and wire too flimsy, but in any event, I'd go to a commercial "truck" shop, specifically, to see if their bender can handle the coping. If it does, then you just need make the bends/radius/length for them to follow.
I think I've still got some clay wheels around here.