IIRC a panel covering the open space between the bottom of the front pan and the K member helps, but it increases oil temp.
You're probably catching air under the trunk, but what to do?
For best effect the dam should be as low as possible, a rubber skirt bottom (I've seen garage door skirts used) gets it closer without being wrecked when the nose dives, curbs, driveways, trailer ramps.
If legal (or you don't care!), it should project forward ahead of anything else, and angle back ending at the radiator core bottom.