box
匣 centers on a container: a box or case for holding things.
匣 combines the enclosure 匚 (box) with 甲, which likely contributes the sound. The character represents a box or case.
The outer shape 匚 is like a box, and inside is 甲, which looks like a lid or a shell. Picture a box with a protective shell-like lid: 匣 is a box.
For コウ, imagine a box that makes a hollow 'koh' sound when you knock on it: koh -> コウ, and the box is 匣.