Nice job on the chain box lids
Rather than using the electric spot welding process, many home hobby people use the plug welding technique where you drill a hole in one side and fill the hole using a Mig or Tig welder , you grind down the plug weld flat , it's a rather messy method but it does work. I used that method on the C8 body , where some of the panels are up to approx. 2mm thick.
These days, the home hobby welders have gone ahead in leaps and bounds: for around 600-70 bucks you can buy a decent quality all singing and dancing TIG welder and weld just about anything including Aluminium. I'm not a fan of the 3 in one welders myself, the Stick, Mig and Tig in one box style of setup, they are a compromise to some degree.