cage, pen for wild animals
柙 centers on a physical enclosure for containing wild animals: a cage or pen.
柙 combines 木 (wood) with 甲, which likely contributes the sound and suggests a hard shell or covering, together expressing a wooden cage or pen.
The left side 木 is wood, and the right side 甲 looks like a barred enclosure. Picture a wooden cage with a cross-barred front holding a wild animal.
For コウ, imagine a wild animal in a cage letting out a low, hollow growl: kou... The sound cues コウ while the animal is trapped in the wooden pen.