Actually I am under doctor's orders to wear a hat when out in the sun. I like the flat cap but it does not give much protection to the ears and the side of the face. I started wearing Autralian leather hats, I have several, and found that they stayed in place, for me anyway, at speeds of up to 140 kph. In the height of summer I prefer a panama hat and when I tried it in the Morgan, in the first instance at speeds up to 70 kmh, I found it stayed in place ok. I have since found that I can also wear it at higher speeds depending a bit on conditions. If I get a couple of frights with it starting to lift I swap it for a leather hat or the flat cap. In any case around town or on restricted roads it is fine.
I have a lifetime supply of hats because I started importing them but in the end gave up because I was paying more in taxes than I was earning