tablet, signpost
楬 centers on a wooden marker used for identification or direction: a tablet or signpost. Both meanings describe a flat piece of wood with writing that conveys information.
楬 combines 木 (tree/wood) with 曷, which likely contributes the sound. The character originally referred to a wooden tablet or signpost.
The left side 木 is wood, and the right side 曷 looks like a person 人 under a cover 勹 with a sun 日, as if someone is posting a sign. Imagine a wooden signpost standing in the sun with a notice attached.
For ケツ, picture a wooden tablet with the word 'KETSU' carved into it, like a nameplate. The sound 'ketsu' is close to 'kets' in English, and the tablet is made of wood.