raincoat
襫 centers on a garment for wet weather: a raincoat. It is a specific clothing term with no broader abstract extensions.
襫 combines the clothing radical 衤 with 奭, which likely contributes the sound. The exact historical development of 奭 is uncertain, but the character as a whole is a phono-semantic compound for a type of rain garment.
The left side 衤 is clothing, and the right side 奭 looks like a person under a wide shelter. Imagine a wide raincoat draped over someone, keeping them dry.
For セキ, picture a rainy day where you grab your raincoat and say 'It's wet, I need my seki-coat!' The made-up word 'seki-coat' cues セキ while tying directly to the raincoat meaning.
—