Not one size fits all
First, let me say this is a great cap idea. It does a great job at letting air flow. The problem is the sizing. I have it sized all the way down and it is still a little loose in the wind. I wear a medium in other OR hats, but this is swimmingly large. I'm looking at replacing the strap design with Velcro, hopefully that will allow me to adjust it to an appropriate size.