Master makes an outboard lock for removable screw-type outboard mounts.
Basically, it's a open box base that you clamp onto the transom using the outboard mounting screws, a box-top then goes over the screw "handles", and a lock secures the top to the base, trapping the handles inside.
I bought one; it was about $40 US (I think.) Never used it, as both my motors were permanently mounted. Haven't found anyone with a valuable enough removable motor to warrant giving it to
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jky