Fence
You've made yourself a tree bookshelf (that is, a bookshelf cut into a tree), so you build a fence to keep the riffraff out.
Your tree bookshelf is often being raided by the neighbors, and since it's outside, all you can do is build a fence around it to protect it. Better check out that fence-building book in your tree bookshelf.
さく
しがらみ
None
On your fence, you hang up a sack (さく) to accept donations.
People put money and books in your fence sack to keep your tree bookshelf well stocked.