I built a new exhaust manifold and mixer from stainless 2 years ago. I made a template from PVC water pipe first then fabricated from off the shelf bends. Worked out well and no issues so far. Bends and tube from Steel & Tube Stainless. Price was reasonable and their rep was very helpful. Had a mate help with welding. Only downside was the time it took but I had no other option as the old manifold was badly corroded and no original replacement available.