On my recent trip of 1,800 miles I saw lots of long haul trucks, about 10 of these had aero covers on the back two axles of the power unit (cab).
From what I saw as we passed the trucks (I know we were going to fast for FE) the caps looked like a manufactured product, with a hole for checking tire pressure.
The first one I saw turned my head as I went, cool large pizza pans like on my car
I only saw these on trucks in Texas, & Oklahoma none in Missouri, Illinois, Indiana or Michigan.