casing, shell, husk
壳 centers on a protective outer layer: a casing, shell, or husk that encloses something.
壳 is a simplified form of 殼, which combines 殳 (a hand holding a tool) with the sound component 壳. The original meaning relates to striking or removing a hard outer layer, hence casing or shell.
The top 士 looks like a lid, and the bottom 冗 suggests a container with legs. Picture a protective shell or husk that covers something inside.
For カク, imagine a hard shell cracking open with a sharp 'kak' sound: kak -> カク, and the casing splits apart.