bark of a tree
樸 centers on the outer bark of a tree, the rough protective layer that covers the wood.
樸 combines 木 (tree) with 菐, which likely contributes the sound. The character originally referred to unworked wood or bark.
The left side is a tree 木, and the right side 菐 looks like a thick covering. Picture peeling the rough bark from a tree trunk.
For ボク, imagine a box made of tree bark: box -> ボク, and the rough bark covers the wood.