Hi, Folks, from upside-down Australia
Martin you started with asking about stainless, and got the replies, Yes, but needs a serious flux. Serious in this case meaning genuinely dangerous (Hydrofluoric acid).
Stainless protects itself from rust by forming an instant impervious oxide film of nickel and chrome oxides and these prevent solder wetting the steel (sticking). There are esoteric ways of dislodging the oxide film with fluxes or scratching it off with a glass brush under a film of oil.
I would not plan to go with that
Then the discussion goes into mild and plated carbon steels, and the answer is still yes, and the fluxes are kinder.
Do some hands-on research on the bin you are considering - take a magnet and see if its magnetic. If it is than its probably one of the Ferritic Stainlesses 430 or similar. (austenitic stainlesses 304, 310, 316 etc)
Any road up - your idea is great - make a hull or boat in panels and join them like the panels of a stitch and glue hull with epoxy (or car body polyester resin) either resin needs to be filled till it is a a paste that will not slump.
I have rambled - hope something is either use or interesting
andrew