rest room, privy
圊 centers on a private enclosed space for bodily relief: a rest room or privy.
圊 combines the enclosure 囗 with 青, which likely contributes the sound. The character is rare, and its exact historical development is uncertain.
Inside the enclosure 囗, the blue-green 青 suggests a clean, tiled rest room. Picture a private privy with blue walls.
For セイ, imagine saying 'say' when you need to use the rest room: 'say' -> セイ, and you step into the enclosed privy.