[ecomodded]

I am not sure how many people have contemplated that idea, I think it would win favor.
If your car has wishbone suspension a election sign would fill the space nicely, You should consider raising your front air dam/spoiler to near the same height of the planned belly pan, to avoid low pressure zones/ empty space.

Filling behind the rear wheels to the bumper well remove the parachute effect caused by the voids.
If you leave your air dam 3" off the ground then a front belly pan would need to drop down to near that level to be effective, or you could even not put a belly pan on since the air will be traveling below suspension parts.
I would add the rear belly pans if i was to keep the low front air dam, as a bonus its easier.