After both corners were welded in I tackled the center patch. Here I've flattened a copper plumbing "T", clamped it in a vise grip and wedged it between the seat riser and the seam. The copper helps soak up the heat. Lots of small tacks to hold things in place. I was very concerned about warpage so I did my best to keep it to a minimum. Welding two flat pieces of sheetmetal together is quite a challenge.