The Lexan is 1/4 inch thick. This was used for a couple reasons. First because it was readily available. Second because this thickness makes the surface of the covers nearly perfectly flush with the tire sidewalls.
There is a 2 inch hole drilled with a hole saw for the valve stem.
I drilled and tapped five 1/4-20 holes for each cover. They are drilled and tapped 3/4 inch deep into the very thick aluminum spokes. This was the best method I could come up with to mount them.
I am a bit concerned about dirt on the inside. If they get dirty and ugly quickly, I will paint them or use them as a template for aluminum discs.
Oh, since I made these and deleted my passenger side mirror, I have done some testing. I am seeing roughly 2mpg increase both on the highway and at 35 to 40mph areas. To give you an idea of the difference, the best mileage I have ever seem for any real distance was 62.3mpg. That was done at 75 degrees outside. Today it was 56 degrees and misty. This would normally relegate my mileage to the mid to high 50s (maybe 57 or 58mpg max). However, in this inclement weather today I averaged 61.9mpg for the 40 miles of my test. This was averaging 35 to 40mph.
I will know more as I drive the car more.
My hope is to finish my grille blocks, lower pan, and chin spoiler and be able to always be above 60mpg in town and rise from the stock 44mpg highway to 50mpg. That is my goal.
We shall see how it works out.
Matt