Ahh, the mighty mounts. I've seen those... pretty cool/convenient, but I think to have fully locking functionality you'd need to use the ones that mount/lock to the OEM track (which, of course, are more expensive and require use of Yakima crossbars).

Maybe you could replace the mounting bolts with something much harder to remove like a torx head bolt, or maybe safety wire the mounting bolt so it would require a thief to have a fairly specific type of tool to remove.

Good luck!