Crawl Space Repair - Attempt to block a foundation vent

The homeowner had the right idea to block the foundation vents. All they really do is let in the nasty summer air. All that humidity is like poison to a crawlspace. The problem is that plywood will soak up moisture and warp or rot. It also isn't a very tight fit, so air still gets in easily. DryZone uses several methods for blocking off these vents. Which method used depends on the situation, are we just trying to block air or would you like to prevent structural damage from floods? (See next picture)