Here is what we did today.
I used an old 4x8 sheet of plywood siding ripped down to 16 inches for my sides. I used an old door frame to get the spacing and shape I wanted around the wheel weels.
There is about half an inch between the plywood and my moon caps.
Our 59.7 Mile trip yielded 47.8mpg on the trip north for a new trip record.
Our trip home yielded 47.0mpg with much colder ambient/intake temps.
No ABA test, but if I had to guess I'd say 2-3mpgs between changing to synthetic fluids and adding the sides.