raft, calyx
柎 centers on a supporting or enclosing structure: a raft that supports on water, or a calyx that encloses a flower bud. Both meanings share the idea of a base or holder.
柎 combines 木 (tree/wood) with 付, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a wooden object related to the sound フ.
A wooden 木 structure that is attached 付 to something: imagine a raft made of logs tied together, or the wooden-like base of a flower's calyx holding the petals.
For フ, picture a wooden raft floating on the water, making a soft 'foo' sound as it drifts: foo -> フ.