bamboo rope
筰 centers on a rope or cord made from bamboo, used for binding or tying.
筰 combines 竹 (bamboo) with 作, which likely contributes the sound サク and the idea of making or constructing, suggesting a rope made from bamboo.
The bamboo top 竹 sits above 作 (make). Imagine twisting bamboo strips together to make a strong rope.
For サク, picture a sack made of bamboo rope: sack -> サク, and the rope holds the sack together.