belly pan for sure. I would consider a partial grill block with some ducting the remaining air directly to the radiator, I agree with kach22i, you don't want to put yourself in danger of ruining your engine. I did this to a lesser degree on my Saturn ion (towed 3500lbs without overheating) and plan on doing this to a larger degree on my gmc acadia.
you might want to make standoffs for the belly pan so you can keep it flatter and cover more of the bottom, this would make a huge difference in stability, noise, and most likely in economy. splitters/front mount air dams (see chevy volt and traverse for OEM examples that work well) will compliment this for sure.
getting the taper just right is a challenge
maybe you can pull angles from this sample diagram? I know for any angle, your rear tapers are awful short, and need to be longer likely for any noticeable effect.
AeroTech Caps | Aerodynamic Wheel Covers, Trailer Skirts and More!
these are some production EPA certified examples for inspiration