Water-proof, I dunno. But water-resistant.... I'd say wool.
But use a tight weave. Maybe thoroughly pre-shrink the fabric (hot wash & hot tumble-dry) to tighten up the weave even more.
If you want to be super zealous there's always scotch-guard....
....afterthought....
Gore-Tex??? I'm amazed they sell it by the
yard! Quick, Esty, sale ends March 19....