outer box for a coffin
槨 refers specifically to the outer protective box or casing of a coffin, not the inner coffin itself.
槨 combines 木 (wood) with 郭, which likely contributes the sound and the idea of an outer enclosure. The character thus represents a wooden outer casing.
The wood radical 木 on the left suggests a wooden structure, and 郭 on the right resembles an outer wall or enclosure. Together they form the wooden outer box of a coffin.
For カク, imagine a wooden outer box shaped like a cube: cube -> カク, and the box encloses the coffin.
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stone burial chamber