In Gibraltar we could only get 1.5 mm thick 30 by 30 316 stainless, I had brought the two shorter parts from the UK, the had a 2mm wall thickness, which was really as thin as you can go with arc welding, so trying to weld 2mm onto 1.5 mm, was not without its challenges, though, eventually, we managed it.
TIG welding would have done a much better job, perhaps even MIG would have done better, but we used what we had, and got strong welds with a 2.5 mum 316 rod, and current varying between 40-65 amps