Check out the nose of vetter's streamlined bikes - it's curved at the nose to allow some air to still get under the vehicle. Not much, but you need enough flow to fill the space between the car and the road, else you get the same effect as the wake behind the vehicle, which will create downforce by means of a vacuum under the car as the air rushes over and around the vehicle, but not under it to equalize the negative pressure.
No matter what you end up doing, make sure it can be changed without too much frustration, because after you test it, you may reveal a weak point in your design, such as a portion with detached flow or a turbulent vortex.
Hey, you never know.
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