Update:
In retrospect I learned: the plastic I used is too light. It wasn't an issue when I made them when it was cool outside (and the plastic was stiffer), but in the peak summer temps it got much softer and the skirts were pretty ... wobbly. I need to glue something on the backside to stiffen them.
If I were making another set (still haven't done my front wheel tests
), I believe I'd go with fiberglass.