Bill wrote:
In message , David
writes
I assume that aluminium would be lighter and easier to work with
enough strength for this job.

When we built the boat almost all the stainless steel, aluminium and
copper sheet came from scrapyards. We just climbed onto the piles of
metal and selected suitable sizes and quality.
Of course, I was a bit younger then, and maybe they were less in thrall
to safety.
I was going to suggest scrapyards, some will allow rummaging if you
wear boots and hi viz vest and will then sell at so much a kilo (or pound)