This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 瓨 is uncertain. It visually combines 工 and 瓦, but the original semantic or phonetic role of these components is unclear.
With no clear meaning, picture a clay vessel (瓦) being crafted by a worker (工): 瓨 might be an ancient jar or pot.
For コウ, imagine the worker (工) making a pot that goes 'kou' (コウ) when tapped, like a hollow sound.