barrel hoop
箍 centers on a tight band that holds something together: a barrel hoop. The idea is a circular binding that encircles and secures.
箍 combines 竹 (bamboo) with 㧜, which likely contributes the sound and the idea of encircling or binding. Bamboo strips were historically used to make hoops.
The top 竹 is bamboo, and the bottom 㧜 looks like a hand wrapping something around. Picture bamboo strips being bent into a tight hoop by hand to bind a barrel.
For コ, imagine a small child (子, こ) trying to roll a barrel hoop: 子 -> コ, and the hoop is made of bamboo.