alcove for an image
龕 centers on a small recessed space, specifically an alcove used to house a sacred image or object.
龕 combines 合 (to fit together, join) with 龍 (dragon), likely suggesting a container or enclosure that holds something precious, like a dragon in a shrine. The exact historical development is uncertain.
Picture a dragon (龍) fitting snugly (合) into a small alcove on a temple wall, guarding a sacred image inside.
For カン, imagine a can (カン) placed inside the alcove as an offering to the dragon: can -> カン, and the alcove holds it.