I blocked all the grill above the bumper and left it open below the bumper. I used coroplast. My engine temperature dropped about 8 degrees F on the average, as read by the Scangauge, after the mod. I live in Clearwater, FL.
I also made my own topper for the back. It is a practical one and not super aerodynamic like many have built here. Here is a rear view picture of it:
It weighs only 138 lb. It has a wood frame covered with 4 mm marine plywood both inside and out, with styrene foam between the plywood. It has a light layer of kevlar topped with a light layer of fiberglass both inside and outside.
I get about 30 mpg around town and about 36 mpg hwy.