This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
庎 combines 广 (building or roof) with 介, but the exact historical development and original meaning are uncertain.
The kanji has a roof 广 over 介, but without a clear meaning, it is hard to build a reliable mnemonic. Imagine a rare, forgotten tool stored under a roof.
For カイ, picture a kayak stored under the roof 广: kayak -> カイ, though the kanji's meaning is unclear.