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Box section aluminum would be the best thing. Otherwise pressure treated wood, or ideally teak and accept it'll warp a bit. With wood prices as they are I'd be tempted to by a 2nd hand teak table or large bench and modify it. It will have also been outside and done most of its warping.
As it'll be folded up in the worst weather you could protect it by making a lip to keep the vertical rain off. Sort of like the way this works:
I'd also get some agricultural style gate hinges with plenty of play in them
Vs
Tighter fitting ones that will seaze eventually:
I'm thinking of building a fold down table in the garden. Attached to the wall so it is out of the way most of the time but a decent size when folded down. Seems like it should be pretty easy with some hinges and a fold down leg to provide support.
Question is what to make it of? Need something that can be left outside all year round without getting manky or warping.