I finally got around to doing something about this:
...that big hole down the center of the car. $15 of coroplast, duct tape, zip ties, and some screen door mesh later, I have
almost a completely smooth underside.
It's hard to see, but I cut a ~6" wide channel under the catalytic converter and resonator and used the aluminum mesh there so it doesn't melt the coroplast. I have one more sheet left to see what I can do on the back end around the torsion bar and muffler.