Crown, Cap
Your forehead is the origin for the measurement for your crown! If you measure somewhere else on your head for a crown it won't fit.
Imagine someone putting a tape measure around your forehead and taking careful measurements for your crown. They better not mess up, or you'll have them put to death!
かん
かんむり
None
You want your crown to be just like Genghis Khan's (かん), which was more of a cap. He's your hero though, because he ruled over the most people, so you want yours to match his measurements.
Why don't you go look up what Genghis Khan's crown/cap looked like. Then try to imagine how it would look atop your head.