Enclosed spaces in a non-velocity flood zone require flood vents. The purpose of these vents is to equalize pressure in the event of a flood event. In the past foundation ventilation products or soffit vents have been used for this purpose. The International Building Code uses ASCE 24-05 as a reference for flood vents. ASCE 24-05 requires that the vents allow a 3″ sphere to pass through the vent. This eliminates almost all less expensive alternatives. The only product that I am aware of which will meets the code requirements is by “Smart Vent” you can check out there products at www.smartvent.com