have you considered perforated sheet metal ?
when I do both my grill blocks and my airdam I plan on using perforated aluminum that allows a 38% air to surface. this way I'll still be letting in enough air to keep things cool (I hope) while giving most of the advantages of a full block off.
my truck came stock with a trans cooler, which sits right below the grill opening, so taking that into consideration as well. I posted a mockup of what Im planning on my grill to give you an idea of what it will look like. in this post:
grill mockup