[Otto]

And, when you get the exit geometry sorted out, you'll find that you can then decrease the size of the inlet, with subsequent improvement in overall streamlining. This because with Bernoulli effect sucking the spent cooling air out the well-designed outlet, it takes a lot less ram air (and consequent drag) to stuff cooling air into the inlet in the first place. Less air will be doing more cooling, more efficiently.

BTW, make sure the cooling air has a nice, smooth path(s) between inlet and outlet.

Google for pics of WWII aircraft, and study the engine cowlings on, say, the B-29. Or, if you prefer a recent example, Google for Arnold AR-5 and have a gander at a remarkably efficient little plane, which despite fixed gear, tractor propeller, and cruciform tail, does amazing things on little power. Study the cooling vent on the side of the cowling, which works great but has no moving parts. Design your cooling outlet on the bottom of your Coroplasted car accordingly.

If you need more air hitting the oil pan (finned aluminum on both my BMWs and my Porsche, for enhanced cooling), then take a carpet knife to that Coroplast and make a nice NACA duct with ~7 degree ramp slope. Bits of Coroplast and some duct tape for duct sidewalls will finish the job nicely.

PS: You did a damn nice job installing that Coroplast belly pan, and once you get the exit done, I'll bet your fuel economy has significant improvement