rafter, place name
枅 centers on a structural beam in traditional architecture: a rafter. The place name sense is a specialized application of that architectural term.
枅 combines 木 (wood/tree) with 开, which likely contributes a phonetic element. The character refers to a wooden rafter or beam.
The left side is 木 (wood), and the right side 开 looks like two parallel beams. Picture a wooden rafter made from a tree, shaped into a long straight beam.
For ケイ, imagine a carpenter named Kay carefully placing a wooden rafter: Kay -> ケイ, and the beam fits perfectly.