reed, guide for yarn on a loom
筬 centers on a tool used in weaving: a reed that spaces warp threads and guides yarn on a loom.
筬 combines 竹 (bamboo) with 成, which likely contributes the sound. The character refers to a bamboo reed used in weaving.
The top 竹 (bamboo) suggests the material, and the bottom 成 looks like a frame holding threads. Picture a bamboo reed guiding yarn across a loom.
For セイ, imagine saying 'say' as you guide the yarn: 'say' -> セイ, and the reed helps the loom say its pattern.