I'm adding a gable vent (non-powered & no fan) on both ends. Roofer recommends something no bigger than 12"x12" in aluminum with critter screen from big box stores. I'm thinking more a rectangle rather than a square. What else to consider...Do they open & close? I would appreciate your thoughts & opinions especially as to type and material. FWIW, this is stucco over concrete block in Dade county, Florida within 1/2 mile of ocean. THANKS!