Matt, 'Funny you'd mention that since I have just this last couple of weekends been trying to figure out how to keep the ladder from floating up and off its bracket while I am swimming. That self stick rubber keeper fell off soon after initial use.
I stow my ladder across the stern sideways by shock cording it against the inside back of the hand rail.
I was looking for a kind of large pin-lock bolt thing to push thru the ladder mount just above the ladder pins, but settled on some 5/8" rubber straps with metal loops on the ends which was an old design to hold coiled ropes. I just cram it into the ladder mount slot and use the metal end loops for a lanyard.
Let me know what you find!
John