if you get a chance, check underneath an 06 jetta diesel to see a really good undercarriage - done through a combination of bolt-on smooth trays & gap filler panels. they put a lot of work into it and it's pretty astonishing, compared to most new cars. and they only did it on the diesel; the gas car has a naked undercarriage.
makes you wonder why carmakers don't offer aerodynamic "options" - i mean, aside from often purely cosmetic rear deck spoilers. maybe the jetta panels cost a couple hundred bucks, but if people knew they'd reduce fuel consumption by 3 or 4% (reasonable guess), maybe they'd tick that box on the option sheet. same for rear wheel skirts. they're not rocket science to add to any car, and they help a few percent.
I already have a tray like that on my car, it looks a little bit different but basically looks like that. It deflects the air from going into the engine bay but has a couple holes to vent the engine compartment.
My main concern is where the engine bay ends from that point on I want to cover everything all the way to the back bumper.
People these days are more interested in looks, not fuctionality. A Rice wing( WRX, EVO) does nothing for performance but it looks nice. Same with all the chrome stuff around the car. That is why they will tick the rice wing, big heavy chrome rims and tint on the option sheet. And then they will complain that they get horrible gas mileage and their car accelarates like a tank.