crown
冕 centers on a ceremonial headpiece: a crown, specifically the type worn by ancient Chinese emperors.
冕 combines ⺜ (a covering or cap) with 免, which likely contributes the sound. Together they represent a ceremonial crown or cap.
The top ⺜ looks like a cap, and the bottom 免 suggests something placed on the head. Picture an emperor placing a distinctive cap on his head: that cap is a crown.
For ベン, imagine a royal attendant named Ben carefully placing the crown on the emperor's head: Ben -> ベン, and the crown is the center of the ceremony.