I am new to the craft of forming sheetmetal. I want to form a vault shape for a new transmission tunnel. I have made a light cardboard shape template and I have a sheet of 18 ga. sheetmetal that I intend to use. The new tunnel shape will be welded to the original floor pan. What method should I use to shape the tunnel to result in a smooth rounded form as opposed to straight line breaks. I was thinking of somethong like a chunk of large steel pipe or telephone pole (neither of which I have) or something else. The method should be low-tech, because I don't have a lot of special tooling. Thanks for any help.