2.7 mm. thick Lauan plywood, primed with oil base paint that was thinned slightly. Panel was cut with a box cutter. Stainless sheet metal screws hold it to the oem plastic bumper cover.
This definitely seems to help, compared to the prior setup:
Coasting is better than before. Recent mpg gains aren't entirely due to the front panel; I also recently switched to 10W-30 oil from 15W-40 after replacing the O-ring seal where cam shaft meets distributor.