My outboard has a retaining eye provided, I believe, for just this purpose, I always use a very short retaining leash attached to it, the other end clipped onto one of the ring eye bolts on the transom.
I also pass a carabiner clip or D shackle through the eyeholes on the two transom bracket clamp screw handles, or now that I have a lock that passes over both handles I sometimes use that, especially if planning to leave the boat unattended since it's a requirement of my (Craftinsure) insurance policy.
I guess if you wanted to be even more secure you could drill through the transom and add a bolt, but would a shake-proof washer and wingnut suffice or would it need a locknut? Since I recall you trailer your boat, then that wouldn't be too much hassle, but for those who fold and carry???