⺮ is the bamboo-top radical, the cap-form of 竹 used when bamboo sits over another component. Indexes 箱 in the bamboo-craft family: containers and tools historically woven from bamboo strips. Same radical anchors 笔 brush, 答 answer (bamboo slip), 篮 basket, 筷 chopsticks, 笛 flute. A box was originally a bamboo crate.
Bottom 相 supplies the sound — xiāng, exact phonetic match. 相 itself is a 木 tree + 目 eye picture (inspecting a tree's wood). 'Inspecting wood' faintly resonates with checking what's in a box, but the role is overwhelmingly phonetic. Same xiāng phonetic in 想 think, 厢 wing-room, 湘 Hunan.
