I assume you have anchor points in your truck bed to place a lock????
1. utilize a smaller cable that would go through where the tow hitch pins gp through.
2. use a larger cabel and go around or between the poles that extend to make the shelter larger. Any smart person know they could just extend the pole and they come apart but most criminals are morons to start with. Just make the cable visible…
Make the cable asvisible as possible to deter would be theives.
Those are my best guesses. also lock tail gate…