I looked through my pile of stringer channel pieces I had made and I didn't find any more that I liked, so I made some new ones and welded them up. Next step was to measure and weld the two standoffs in the middle between the front mount and the mounts on the tail. Once those are in place, I can start forming the stringer material. I found a trick way to hold those standoffs to the tube. I just used a large hose clamp and it held it in place just great. It didn't need hardly any tension at all to keep it in place while tack welding it.
