Matt,
I was thinking about this driving down the Southfield Freeway today...
Not sure how 'flat' of a surface you have facing outward on on your C-Max wheels. But if you have some flat surfaces, maybe look into
3M Dual Lock. It is like Velcro, but better and more rugged etc. Each side is the same as well. It is used in industrial applications, and we use it to mount expensive test equipment inside the cabs.
Then perhaps some coroplast or acrylic or even cut the bottom out of a plastic oil drain pan to use as discs.
Also...
Here is a fabric based one we use at work on our Freightliner Cascadia test fleet. Supposedly, during testing, they got approx. 0.25% better FE per wheel.
Sometimes they only put them on the 4 tractor wheels, for a 1% gain.
If they have it on all 4 tractor and all 4 trailer, they are getting close to the gain the manufacturer states, for a total of 2% FE gain.
Aerodynamic Wheel Covers | Tractor Trailer Wheel Covers | Deflecktor
Cheers,
Sean