On one project I did I used cardboard tubing of the approximate size I wanted to use and taped it all together firmly to show the routing of the exhaust system I wanted. It had the bends, a couple places for connections, etc. I took it to my local muffler shop in a few pieces and they bent it up for me in a couple hours. Melvin (the shop guy) said, " I've never seen that one before." It worked well enough for me. Plus it was fairly cheap.
