The factory chin spoiler on my truck had broken into several pieces years ago, and the duct tape holding it together had been replaced several times, so I decided it was time to make a proper air dam.
I wanted something slightly lower than the factory chin spoiler, plus it had to be sturdy enough to smash into a snowbank or plow pile without damage. So I bought some conveyor belting, stainless carriage bolts, silicon bronze nuts and washers, and built an air dam. The bottom row of bolts fasten to angles that bolt to a second piece of conveyor belt intended to add stiffness to the visible piece. I have not yet observed whether the air dam flexes back under wind loads on the highway.
Unfortunately, I cannot properly test it. Any MPG difference from the factory setup will be small, and my driving habits will change since I retired last week.