I'd probably make my own out of BS1088 okoume or meranti ply. Cut 45º angles for the joins between boards, with the angle down and out, to shed rain. Probably thin angled router cuts with plunge router, again, down and out, with screening attached to the inside. And I'd varnish them. Alternatively, you can get SS louver grilles that you could install in the boards. (I don't have a plunge router, so I'm not sure how viable angled cuts to create louvers would be, and to some extent would probably depend on the thickness of the ply, with thicker ply being more effective at keeping out rain.)