flat, round split-bamboo contain
匾 centers on a flat, shallow container, often made from split bamboo. The idea of flatness extends to describing something thin or flat in shape.
匾 combines the enclosure 匚 (container) with 扁, which likely contributes both sound and the idea of flatness. The character originally referred to a flat bamboo container.
The outer 匚 is a container, and inside is 扁, which looks like a flat, split bamboo object. Picture a flat, round bamboo tray used for holding things.
For ヘン, imagine a hen sitting on a flat bamboo tray: hen -> ヘン, and the tray is flat and round.