Currently on my wish list is a set of underbody panels. The rear diffuser was designed by a team of engineers and wind-tunnel tested to reduce drag, and the rear suspension covers need to go with it. I'd consider making the forward panels out of coroplast to save money, but considering they'd be covering the exhaust, it might not be the best idea.
Here's one set:
Here's another, without the diffuser:
Panels C and D from set #2 can be used in conjunction with set #1, though I imagine it's well into diminishing returns. More, supposedly the team that designed the black panels left some spaces open intentionally, because otherwise there can bee a lot of resonance from the exhaust and driveshaft that gets trapped in the tunnel, resulting in drone on the highway.
Euro spec cars have these two small black plastic panels that American and Japanese cars didn't get. I might drive by Mazda and see if they might be a more economical choice for the middle and front of the car, than having stamped aluminum ones shipped from Europe or the US.
