one of these.
But coarser weave than shown; air flows through; if it's Earth 2020-HOT: can douse the sucker with water periodically--evap. cooler.
It's light/not crushable-for-pocket like the kepi, though. (Offers some protection, if you're trying to play the cornet at a gathering ... and they're throwing [small] tomatoes? bonus.)
Have seen versions with tiny fan--now solar-powered, of course