The screw handles that secure the motor to the transom on most outboards have a hole drilled in each of the two turning screws. When turned toward each other, the distance of the two holes is about one and half inches. Many padlocks can fit both holes. Spray some corrosion block in the lock to prevent corrosion and lockup. This is a cheap simple solution. It isn't as secure as some store-bought locks - but it works in a pinch or semi-secure area (or short period of time).