Typically, the piece that covers the underside of the engine bay in the rear is referred to as the diaper”.  This is made up of 2 plastic pieces that cover up the engine pieces and prevent some splashing upward from puddles.  They also help to direct air in some cases.  C-One makes replacement panels that help to keep the underside of the car nice and flat.  It also has some fins that create a rear diffuser.  All of this helps out in the aerodynamics department.  They also look pretty cool.

When I bought the under panels, I got them from a forum member that had the panels, but not the mounting hardware.  After some searching, I was able to find the mounting hardware at a shop in Orlando.  Turns out Beatrush makes the under panels (and mounting hardware) for C-One and also sells them as their own.  So I ordered the hardware and was ready to install.  Installation was pretty easy.  Remove the diaper.  Bolt up the panels and forward brace.  All of these devices use hex bolts that use a 4mm driver.

Hardware:

Panels with Hardware:

Once installed, it sits nicely tucked away, but can still be seen.  Installation took about an hour with raising the car and taking my time.

Instructions:

Installed: