just run a cable through the handle and connect it to the dock, using two brass locks and a 15-foot braided cable like this. The lock at the dock end should be a long-shackle Sesamee so you can get the shackle through an open dock cleat or eye. If no hardware on the dock, you can wrap it around a piling or dock timber, and lock it to itself. This isn't theft proof, but it makes it more inconvenient to steal. You only need to lock the motor; they'll likely steal someone else's before they take your dinghy without the motor.