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It's cotton duck. It's already treated with parrafin wax. Also the outer layer is designed to swell when rain hits, closing the small gaps in the cotton.
https://www.carradice.co.uk/blog/cotton-duck-the-original-performance-fabric/76
They're pretty waterproof in my experience. The smaller Super C Audax is a good size too if you don't need to carry too much.
TheBonk
Can’t say I’ve ridden it in much rain. It’s only heavy cotton. Suppose you could wax it or alternatively, attach loops to a big enough similar shape bag and use the Carradice bagman rack as support