Originally Posted by mkodama

Design small flange that goes around the perimeter of the vents on the underside of the hood, you may want this anyway just for rigidity. Design a black ABS plastic cover with 3 or 4 barbed tabs that latch onto this flange hold it in place; you could send the design off to an injection molder and probably get them for like $20 each. Use some adhesive backed foam strip on the cover to seal out water and prevent marring or rattling.

add huge HPF markup and BAM! $300 sealable vent option that people would buy just to keep pollen and little drops of water off the top of their engine.