While not protection for the trailer, what could be fun is to make a fake light connector. But short the hots to the ground wire. It isn't actually connected to the trailer wiring, just a "dead plug" for thieves tow vehicle.
Then if someone tries to take the trailer, plug in the connector and hit the brakes, it blows the fuses on the tow vehicle's brake and possibly turn circuits. Turn on the lights, poof there goes the tow vehicle tail lights. If a 7 pin connection, you could really cause havoc, possibly blowing out alternator diodes and brake controllers.
This is what happens when I haven't been working hard today.
dave