This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 卥 is uncertain. It may be an ancient variant or a rare character with unclear origin.
Since the meaning is unclear, focus on the shape: a top element ⺊ over an enclosure 囗, like a lid on a box. Remember it as a mysterious box with an unknown content.
For セイ, imagine a sealed box with a 'say' label: say -> セイ, as if the box is telling you its secret reading.