Get some sheet stainles (aluminum could work too), cut out a frame the shape of the cutouts (something like a tombstone), my eye says something 3/4" or so in width. Then get some of that woven stainless wire mesh we often see in grilles. Polish to a bright shine. That way they'll look like intentional "hi tech" vents to releave under chassis air pressure.

Not trying to rain on anyone's parade, but I think lights that low are a potential problem. Following drivers are inattentive enough without putting the lights down so low that they're even more difficult to see.