Update on how I mounted my diffuser, as it didn't come with any hardware.
Clamped it under the body where I wanted it to sit.
Drilled 4 evenly spaced holes about 1/8" diameter through the diffuser and the bumper cover.
Used 4 of
these screws that are used all over the under tray, and
these standard nut clips.
They're not clipped to anything. Just lying flat on the inside of the bottom of the bumper cover.