You need an indoor /outdoor fabric such as Sunbrella or the mildew will drive you nuts.

I would do two panels, account for a width of at least 50% greater and tie back on both sides when not in use. I doubt when pulled together.....you will get that wet : )

Failing that? When pulled flat across? That would make me a bit crazy. No fullness. Just a flat fabric "wall". Ugh

Better than a curtain by far? A custom drop down roll screen in a rain proof fabric

"Reef Roll"

Available in two fabrics and numerous colors: Braidd5®and Purishade®-reduces the amount of solar heating by 85 percent. The first is a breathable microperforated fabric that can give airiness to the room. The second, a waterproof but breathable fabric, also used in our shade sails.

Reef Roll standard is available in over 50 size combinations, from the smallest 59" x 59" to 157" x 137". You can choose from 13 sizes, 2 types of fabric and 10 different colours.
If you need a tailored one:

https://www.maantaoutdoor.com/exterior-roller-shades/14367-18743-reef-roll-custom.html#/396-material-braid5_breathable/601-color-braid5_cloud_white.