I can't wear hats (except soft floppy ones) because my head shape is more oval than round and the brim of the hat presses too hard on my forehead while the headband has a gap near the ears (results in headaches and/or painful pimple outbreak on the forehead).
I have seen a precision gauge before that has many movable pins that will measure not only the size of your hat, but the exact shape. Making a custom hat based on the measurements of such a gauge would be the only possible way of me being able to wear a hat comfortably (looked for a photo of the measuring device on the internet, but no luck on finding the instrument I mentioned).