I'd go with the metal sheeting or with more fly screen as you suggested. I wouldn't waste my time with plywood, even what they call "marine ply" is rubbish and it will just rot.
Those metal sheets are roofing sheets with a brand named coating called Zincalume. You can buy it in two trays wide as can be seen in your picture, and also 3 trays and 4 trays, it sort of took over from the old corrugated iron. This roofing is especially popular in commercial buildings, if you talk to some builders they'd probably be able to give you enough offcuts to do the job you want.
Side note: It always spins me out seeing houses made of timber, most of Australia has termites so everything's made of concrete and steel.
Those metal sheets are roofing sheets with a brand named coating called Zincalume. You can buy it in two trays wide as can be seen in your picture, and also 3 trays and 4 trays, it sort of took over from the old corrugated iron. This roofing is especially popular in commercial buildings, if you talk to some builders they'd probably be able to give you enough offcuts to do the job you want.
Side note: It always spins me out seeing houses made of timber, most of Australia has termites so everything's made of concrete and steel.