Locking block for Scotty Downriggers. Pair it with a padlock, and it will prevent someone from easily removing the bolts. Recommend printing with a stronger filament, something fiber reinforced just to make it a little bit harder to break. I used PETG-CF for these. Thick walls and heavy infill for more added strength. It won't be unbreakable by any means, but neither is a padlock. It's not going to stop a thief, but will slow them down enough to make them think twice.

Note: In order for this to work correctly, a properly sized padlock is required. Too small of a lock will not push snug against the knob and will allow it to spin freely. You will need a lock with a 7mm (9/32") diameter shackle. Here are the padlocks I am currently using.