Hi Dave,
There is substantial lugs available in stainless. Fitting a small length of solid SS in the lug bore and tube bore shouldn't be an issue and using dissimilar welding rods on the join would be the way forward.
https://www.heamar.co.uk/cable-lugs/79405-klauke-87v16-tubular-cable-lug-70-mm-m16-stainless-steel.htmlOtherwise, there was an article in the Star less than a year ago with some really good home made formers in a rounded C shape, cut into a lump of steel. Using one of the formers against a flat lump could press out something you could use. Jan 2021, page 21 thanks to Mike Turner.
Best wishes...