Through initial drawings I quickly settled on a design that could easily be removed from or attached to a sanding block. This design simply sits on top of the sanding block and screws in with four wood screws.

However, in my drawing and ideation process I was unable to settle on a proper palm/hand grip. I drew a simple oval and a hand-shaped imprint, but in reality decided that the final grip shape would need to be somewhere in between, to allow the block to suitably conform to your hand while still fitting many different hand sizes, and even left-handed users.