The "best" setup I've seen on Moon caps is the setup where one drills and taps the alloy rims for the 3 mounting screws. You can then drill the cap for the valve stem. A close second, and less damaging, would be to use steel wheels.
Undertrays do a lot of good, don't show, and can be removed when you sell. You can use inexpensive coroplast in the middle. I prefer aluminum in the front and around any exhaust parts.
The right side mirror can be removed legally in some states, and that makes a small difference.
Rear fender skirts could be made of coroplast and painted with Krylon Plastic paint, or of aluminum and professionally painted at a body shop. Use cardboard first to make and trim a template to size and shape. Saves material
The nut is always the most important element in fuel economy!