concave channels of tiling, long-necked jar
瓴 centers on a ceramic vessel with a long neck, used for holding liquids, and by extension the concave channels of roof tiling that guide water like a jar's neck.
瓴 combines 令 (likely phonetic) with 瓦 (earthenware), suggesting a ceramic vessel. The exact historical development is uncertain, but the phono-semantic structure is plausible.
The left side 令 looks like a person kneeling under a roof, and the right side 瓦 is a roof tile. Picture a long-necked jar made of clay tiles, with its neck curving like a roof channel.
For レイ, imagine a ray of light shining through the long neck of the jar: ray -> レイ, and the light reveals the concave tile channels inside.