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:
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: