I would just do a lap joint with with silicone and rivet in place. A lot less work, and no chance for distortion. Mike
After 20 minutes with the welder, I found I have several helmet problems, which made things difficult for me. One, my decent hood, is defunct - due to battery leakage. Cleaning the battery compartment and putting in new batteries did not fix it. It doesn't auto-darken at all under arc. I was suspicious about that, but this it the first time I confirmed it under an arc. The cartridge seems to be obsolete, or I haven't been able to find a replacement. Kind of mad about that, spent real money on it.
My crap HF helmet (solar powered) does seem to work better than the formerly good hood, but I have to overcome some of it's deficiencies. One, it tends to fog internally, which is awful, and two, the top of the hood doesn't block overhead light, because it is cut back to save 10 cents of plastic. The overhead light comes in AND illuminates the inside stupid white sticker and screws up your vision. I could make a black hood cover to prevent that, but it doesn't make the actual helmet any better. The vision quality isn't suitable for my eyes, or this kind of close work.
Surprisingly, even though I couldn't see well, I made a row of tacks. I didn't fill it all in, but I could see it would be mostly ok. I welded the coupons together with sheet metal butt welding clamps. There's like a 1mm gap between the coupons, which seems to be common for these clamps. But this was only 3" and I have to do maybe 60+". I don't want to spend that level of effort on this - it's not a custom rod, it's a simple lathe backsplash. If I were to entertain doing anything like this, I'd have to drastically upgrade my hood, and do something about hood light leakage even on an upgraded one. To do that much welding requires a lot of time, and the ability to see what you are doing!
So butt welding this is out. On to Plan B. The materials are large enough to be overlapped. I can rivet this together, holding it with cleco clamps, but I have to close the corners. I could tack the corners and then rivet something like an angle on the back side. It would be functional, so there is that... At this point, I will be satisfied with functional. That way I can use my lathe again...