Hi Noah,
The steel wheels allow you to have smooth wheel covers much more easily than alloy, and yes, there was no discernible change when I used the steel wheels. My alloys are not particularly lightweight, in fact they may weigh more than the steel wheels I am using. I may be buying a second set of steel wheels so I can leave my winter tires and regular tires mounted, and do the swap myself. If you use two sets for two years, you will break even, or so on the cost.
I bought the 4mm Coroplast which is the most common thickness. I don't think that 10mm would be as practical for most uses.
I thinned down the curved edges of the rear wheel skirts so they lay close to flush. I left the corrugation everywhere except from about 3/4" from the curved edge. The rod is an 1/8" diameter steel rod and the rust on it keeps it in place. I have it slightly bowed, though that was to clear the domed wheel covers. I now have flat aluminum (16" pizza pan) covers riveted to the plastic wheel covers.
This summer was a big jump forward in FE for me, and I think a lot of it came from the flat wheel covers, much more EOC, and a few tweaks on the rear wheel strakes. My best tank average was over 55MPG!
CarBEN EV5 is actually starting construction, only just started. I'll proceed more as I find the time to run sheets through the PhlatPrinter 3 -- it takes 2-3 hours to complete cutting one 2'x8' sheet which yields a bit less than two 2" thick sections. There are about eighty five 2" thick sections, so one at a time will get it done.